My DD is 12 and is being treated for severe anxiety with CAMHS.
One thing I've noticed is that when the few friends who know what's going on have heard her symptoms, they are suddenly convinced they have it to and need CAMHS referral 
I'm not saying that your daughter isn't suffering with anxiety, far from it, I would however be very cautious about her discussing/reading it online as others experiences could heighten what I'd probably a "normal" episode of anxiety that most of us experience.
Does that make sense?
If you want advice I found Young Minds to be absolutely fantastic. They're a parent advisory charity and can guide you/hand hold you through this and offer a professional input, if required.
On a practical level, see how she is over the next month, try and keep a diary of moods and any obvious triggers. If you go to the GP this will help, just in case they query PMT/hormones.
Day to day: try and wean her off the peer forum, it could do more harm than good. See if MIND have any useful online resources she could read.
Let her know you are there, listen. Not everything that is a big deal to her will seem important to you but it shouldn't be dismissed.
Understand you can't fix everything for her. You can hold her hand and be a shoulder to cry on.
Time to get worried and seek GP help is when it affects her usual routine and she stops interacting on her normal level.
My DD has been out of school since late last year. So it's an extreme, but hopefully my ideas help a little. I know how lonely it is as a parent and how scary.