Upvote:3
You can still see a doctor now, usually there is a time limit of a few days. The doctor just needs to verify that what he/she observes is consistent with your your description of your medical condition preventing you to go on your trip a few days ago. The doctor is not required to prove that you indeed suffered from that specific illness. You may be well now, but sickness usually doesn't last a long time, so there isn't a consistency problem here.
This is still useful for the insurance company, in a case where the insured suffers from some chronic medical problem who likely knew at the time of booking that he might be unfit to travel, would not get compensation.