I have a question...

I'm sorry but this is a spot for those applying to 大学 to be a doctor, not actual doctors. The アドバイス spot may have been a better place to post.
Does she eat alot? Does she skip alot of meals? Are あなた sure she hasn't been ill recently? If あなた realise that she hasn't been eating alot, and she is skipping meals, then i suggest that あなた should tell your parents. Anorexia is a serious illness and if she does have it she needs help, before something bad happens. I hope this helps, and i hope if she is anorexic that she gets better :) heres a website that may help link