The GP should take anything seriously BCBG, be it depression, hormones/PMT or a mixture, I would not necessarily isolate one from the other, especially in teenagers, I think they can be very interlinked. When you think about it, teens are changing rapidly physically and becoming adults mentally also. Add to that todays world of constant news, social media etc etc, it's hard to get away form catastrophies anyway (I mean just look at today's news).
As an ex nurse we were taught to assess people's conditions on how it affected them day to day and with their daily activities. I would take a similar approach here, how do DDs issues affect her day to day/ Does she eat ok? Does she sleep? Go to school, do homework ok? Does she have hobbies/ passtimes she can enjoy (esp ones to help her relax and stop stressing? Does she like watching any comedies, laughter ( is the best medicine they say and it's prob very true. My DD loves comedy DDs and shows esp more old fashioned ones and I think just having fun like this really helps these problems. Other ideas are board games, reading , some sort of exercise.
My DD was awful for a year or two, I bought some Complete Woman capsules to try and balance her hormones and give her iron, as she is vegetarian too. She def seemed a lot worse at certain times, as if part of her cycle and she used to stress terribly about growing taller and becoming an adult but all that has settled. Everyday I had to have long chats with her about her worries. Do you feel able to do any of this yourself? I know it's not easy, I probably would have ended up at GP but for my own medical background and a relative who is a doctor was able to advise re worrying and coping strategies.
I would just be a bit careful that the doctor does not just 'wade in' with counselling unless it seems really indicated. They should take time to assess her properly. Definitely a chat about the periods and how she is feeling on the whole, perhaps some bloods tests and see where you go form there.
I hope I have made some sense here, I had a lot of ideas to get down!
Hope things settle a bit more soon OP, I still don't feel out of the woods with my DD by any means so I can totally sympathise. 