After samadhi what?

So let me get this straight' you have connected to a higher intelligence with a higher understanding and vision and you are asking us the servants of Ego whats next?
Maybe you should ask that question next time you experience that state and give us the answer.

Good question! Many would dwell in their barkwood, "emptiness", if they have gained, taking it for heart wood.

Aside that concentration can be wrong concentration, based on wrong view, leading to wrong release:

from: Maha Saropama Sutta: The Longer Heartwood-simile Discourse

"Just as if a man in need of heartwood, seeking heartwood, wandering in search of heartwood — passing over the heartwood of a great standing tree possessed of heartwood, passing over the sapwood — cutting away the inner bark, were to go off carrying it, thinking, 'heartwood.' A man with good eyesight, seeing him, would say, 'Ah, how this good man didn't know heartwood, didn't know sapwood, didn't know inner bark, didn't know outer bark, didn't know twigs & leaves! That's why he, in need of heartwood, seeking heartwood, wandering in search of heartwood — passing over the heartwood of a great standing tree possessed of heartwood, passing over the sapwood — cutting away the inner bark, went off carrying it, thinking, "heartwood." Whatever heartwood-business he had with heartwood, his purpose won't be served.'

"In the same way, monks, there is the case where a certain son of good family, out of conviction, goes forth from the home life into homelessness, [thinking,] '...Perhaps the end of this entire mass of stress might be discerned!' Having thus gone forth, he encounters gain, offerings, & fame. He is not gratified with that gain, offerings, & fame, his resolve not fulfilled. He is not intoxicated with that gain, offerings, & fame, not heedless about it, and does not fall into heedlessness. Being heedful, he achieves consummation in virtue. He is gratified with that consummation in virtue, but his resolve is not fulfilled. Because of that consummation in virtue he does not exalt himself or disparage others. He is not intoxicated with that consummation in virtue, not heedless about it, and does not fall into heedlessness. Being heedful, he achieves consummation in concentration. He is gratified with that consummation in concentration, his resolve fulfilled. Because of that consummation in concentration he exalts himself and disparages others: 'I am concentrated, my mind at singleness, but these other monks are unconcentrated, their minds scattered.' He is intoxicated with that consummation in concentration, heedless about it, and falls into heedlessness. Being heedless, he dwells in suffering & stress. This, monks, is called a monk who grasps the inner bark of the holy life, and with that he falls short...

So best to gain and put effort into gain of right view, so that such question does not arisen right effort! Again and again and focus proper, yoniso manasikhara, on arise and decay to get disenchanted and gain release.

Worthy to note: even one dwells heedful but having not gained right view yet, one is still an outsider. But even one dwelks heedless (not doing Jhana) but having alreadygained right view, is called an insider (Noble One). That is why a teacher of right view, ne possessing right view (e.g. a Noble One), is regarded higher as a teacher of Jhana. So if avaliable, don't seek for "meditation" teacher (because it brings pleasure) but for right view teacher, which can be of cause painful, according to your character and past. Being able to travel, possessing such skill, does not lead to release, goes not beyond the Cosmos.

As he was standing there he said to the Blessed One: "Is it possible, lord, by traveling, to know or see or reach a far end of the cosmos where one does not take birth, age, die, pass away or reappear?"

"I tell you, friend, that it is not possible by traveling to know or see or reach a far end of the cosmos where one does not take birth, age, die, pass away, or reappear."

[Note: This is a gift of Dhamma and not meant for commercial purpose or other low wordily gains by means of trade and exchange.]

Eckart Tolle describes what I understand as a spontaneous awakening in his book “The Power of Now.’ His experience was not within the context of Buddhist meditation practice, and for me it misses the connection and explanations of an individual ‘experience’ to Buddhist teachings. Though American Zen Buddhist often adopt the ‘no scriptures’ rebellion of monks forced to endlessly recite and memorize scriptures, I believe study of Buddhist scriptures is beneficial to American Buddhists in order to build framework of reference for walking the Buddhist path. In short, it might be good to look to Buddhist teachings to put your experience in that framework. Then again, you could work with your experience within any frame of reference, or none or none.

Show compassion for others. If you really have achievement you can benefit others very easily.

Beyond any doubt you have had an extraordinary experience that has the marks of Enlightenment as defined by Theravadin Buddhism. However, I doubt your experience was that of samadhi (which sometimes follows Enlightenment), because you clearly had some insights, such as “Like I was in deep sleep since I was a kid and I finally woke up,” “I felt like everything including me were connected,” “I was looking the world outside and everybody was like zombies of their own minds. And I realized that I used to be like them too,” “It was like I was sleeping in my entire life and I finally woke up in the real world,” “I was looking the trees, the birds and the fountain like I had never seen anything like this before,” and “I realized that everything is much more than we understand.” Where does it come from? From a traditional point of view, it had to come from having had similar experiences over many lifetimes. Why? Your core intelligence (Bodhicitta) has the function of bringing you up to speed asap. Very few confused or maladaptive sankhara stood in your way, allowing your core intelligence to cause you to experience key insights that were already present in your core intelligence, which is ordinarily unconscious. What is next? The traditional Theravadin answer is you study the Abhidharma. For more information about that path, you can read by book, The Path of Love.

While I have never had such an experience that lasted for days, I have had some rather zen moments upon reaching epiphanies during contemplative thought. I think the longest was about 15 minutes long and happened when I realized the true nature of the universe.

Again, not during meditation but during a period of "focused unfocusing". I would simply relax, get rid of all external stimuli, and play a ringing bell tone. Once there I'd follow any thought down its' rabbit hole and see where it leads me.

No single procedure regarding resting and focusing the mind covers all the ways it may occur. You may have your own self-learned method or stimuli that brings you to a similar or (obviously) greater point. Consider me suitably impressed, but not surprised. We all have our particular strengths - maybe this one is yours?

I'd certainly explore it further. No matter what your faith or beliefs being able to focus your mind in such a manner will have a myriad of benefits.

Learning things about your mind is a significant step forward in humanity. Congratulations!! Enjoy the ride.

So... what was it, where it came from and why?

What you have described sounds very much like the experience of Udayabbaya ñana or the knowledge of Arising and Passing Away. You can search for these terms and figure out more about this. You can also refer to this description of this stage by Daniel Ingram.

As to where it came from, only you can answer that question. Something you undertook, like self-enquiry might have resulted in this. Generally these things are unclear at that time but with sufficient understanding of oneself and the world, it can become more clear.

Why did it happen? Because it is natural, it might present itself to any one who looks inside. For some it may not happen with years of practice, but for some others it can happen by just looking at things with mere curiosity. There could be various reasons for this, ranging from past karma to genetic differences, but its not worth pondering too much on these as these are outside one's control anyways.

what's next?

Spectacular as it may be, it is not the final stage or end result of one's insight practice. In fact, some texts like Visuddhimagga refer to it as the beginning of actual insight practice. The long term goal is to use this to reach stream entry, or the first stage of awakening. To get there you might need regular practice and most probably also guidance from an experienced teacher.

In the short term, it is better not to hold on to these experiences, as it is transient and not repeatable. The clarity might not remain same and identifying with past experiences can cause added suffering. As clarity fades, you might find yourself in difficult territories, so learning and practicing letting go will probably help a lot.