What are the early signs for a person developing agoraphobia?

Between the ages of 17-22 I would get very anxious while travelling, during classes and church. I'm not so bad now, but sometimes still get anxious.

Most of the time, I only go out shopping if I really have to.

I'm not trying to self-diagnose (I feel that I'm over the worse), but I'm asking mostly out of curiosity and also so I'm better informed to help myself if things start to get bad again.

Thank you.