oh man.

bcps regulate estrogen and make you able to have periods again, which i'm assuming you haven't because of malnutricion. Having periods will help with getting your body back regulated.
You should definitly see your doctor and/or a dietition and get their recommendation.
I had an eating disorder aat the beginning of high school but didn't lose terribly much weight becuase of the pcos-relatedinability to lose weight! I skipped a few periods but never had my hair fall out. Definitly eat healing foods full of protein and nutrients [black beans, brown rice, fruits, vegetables, salmon, blahblahblah you probably know this already) to help renourish your body and help your metabolism regulate again.
good luck and much love