Is there something wrong with her?

I'm worried about my girlfriend. She is 18 and I used to think she was just moody and easily irritated but now i wonder if it's something more serious. She can't stand being in social situations because she doesn't 'like people' or feel comfortable with lots of people around sometimes even our friends. Every few days to a week she goes through a period where she feels really down and she goes to sleep each time so she doesn't have to deal with it. Her eating habits seem to fluctuate quite extensively aswel and she occasionally breaks down into tears for a really small reason or for no apparent reason. I know she used to get bullied and has low self esteem and stuff but I'm wondering if it's something like anxiety or depression? should I take her to a GP?