Buttercup, it sounds like it was a tough day when this happened, and with you feeling fragile as you said, you weren't in the proper frame of mind to deal with it. I don't agree with your Head of Year discussing her issues with you in the middle of a crowd...that's extremely unprofessional, and I can't believe that any teacher in their right mind would do a thing like that. She was wrong to single you out in a crowd to discuss a personal issue, and I think she owes you an apology for that.
That being said, it sounds like you have a lot of stress in your life right now. Your health isn't great, you're emotionally fragile and (no offence) you're a teenage girl. That means hormone issues on top of everything else. I really think that if you're not currently seeing a counsellor, you might want to consider it, so you can learn how to manage your stress and anxiety, and your inability to "let go" of issues when they've happened. The longer you allow that stress and anger to boil, the worse the explosion is going to be when it happens...and it will happen.
Big hugs to you Buttercup. Everything will be okay in the end.
DX: Fibro, Severe Myofascial Pain, Chronic Pelvic Pain, Surgical Adhesions, IBS, IC-PBS, Carpal Tunnel (both wrists), FAI, Reynauds, Opthalmic Migraines, Drug Related Hot Flashes, Physically Unable to Vomit due to Nissen Surgery, Extremely tiny and scarred veins (hard start for IV's)
Meds: Oxycontin, Tramacet, Cymbalta, Cesamet, Flexeril, Clonidine plus Vitamin D + Multi daily