If I undergo baptized am I still Catholic?

There is a few parts to this simple question, but the simple answer is "Yes, you are still a Catholic."

Some one suggested to talk to a priest about this and that is the best way to confirm you are still a Catholic. However, the thought that you are still (I assume) willing to talk to a priest means that you are still a Catholic. Just like many other things we do, this may be a sin (talk to a priest they would know and assess best...though it doesn't sound like you had intent or full knowledge of the situation and even so it might only be a venial sin).

As with any sin, we may turn away from God for some time but we are always welcome back once we admit wrong, and we are even welcome as we repent and make reparations. The Catholic Church takes the same attitude, and we and we may sin against her, but we are always welcome. Just because you sin doesn't make you not Catholic, it just makes you a mortal human. The Catholic Church is filled with mortal sinners trying to do better everyday, so you are right at home with everyone.

I just mentioned sin a lot, and in order for this to be a sin, you had to have full knowledge of what is happening, full knowledge of how it's wrong, and willingly choose and intend to do this action. Not having all these components means that you may still have made a mistake but may not have sinned or it may just be a venial sin. Again this depends on what exactly happened and you intent and knowledge. A priest could discern this the best with more details that you may want to share.
Based on my limited knowledge of the situation and your knowledge, it does not sound like you sinned at all. You didn't know and didn't fully participate with full knowledge and you definitely didn't participate to spite God, the church or someone, let alone intend to do harm with your participation (or know of any possible harm with your actions).

In order to NOT be a Catholic anymore, in a nutshell, you have to publicly state your intent. Some write a letter to the bishop with this intent. Others in a different drastic way will publicly denounce their religion possibly by speaking ill of it or by promoting heresies.. Even at those points, those people are still welcome back, though at that point they may possibly not be known as a Catholic anymore.

As a side topic with this, there are people who haven't been to mass in 40+ years and are still technically Catholic. There are people who started to go to a Protestant church after going to. Catholic church for 40+ years and they are still Catholic, technically. And there are people in between both and other situations, who are all still Catholic. Some of those situations are sinful, all of whom we need to pray for and welcome back.

Depending on who they were and what they did, the "baptism" is probably nothing more than a blessing.
I know of one group where they will "baptise" everyone they see and come in contact with and even people sleeping. I forget the name of that group, but all they are doing is sprinkling "holy water" or "holy salt" (I use those terms loosely as they are blessed by their church leader and not a minister with spiritual authority)., And while sprinkling they say a basic informal prayer for you to accept Jesus I hour heart.
I know that many Protestant religions don't believe in baptism (ironically enough Baptists don't) and some have something similar they call a christening....which is very similar to a baptism but without a sacrament.
There is only a few that believe in baptism and call it such and perform it in a similar way as Catholics do. The very few will do something similar as to what I just mentioned with people blessing others. You, possibly being curious about it and asking as they go through the motions and prayers isn't necessarily sinful and isn't enough to get kicked out. If you were just sleeping (or resting) as they did a "drive-by" baptism I don't think counts for anything really, and really is not even a blessing. Knowing more about who these people and their intent would be helpful...we're they the Church of Satan trying to have people denounce Christianity and be baptised in their church, or were they a 7th day Adventist just trying to bless the world through their version of "baptism", or what we Catholics might call a consecration.

A baptism is similar to a blessing and there is more to it, but in general you can only be blessed by someone whom you allow and by someone whom has authority over you. A baptism, as a sacrament and as a blessing with more (as an over simplification), means that they would have to have spiritual authority over you, and that would only be valid if you gave them that authority by denouncing your religion and willingly joining theirs.
Spiritual authority is a big deal when it comes to blessings and sacraments. Without that spiritual authority it would not be valid.