My daughter aged 17 yrs has not been getting her monthly periods since june last year. she had lost a lot of weight. what could be wrong?

Nutrition?: Is she eating enough? Is it possible she has anorexia? Does she have chronic diarrhea or perhaps a thyroid problem? Poor nutrition can stress the body in a way that it "shuts down" it's reproductive capacity (menstruation). She might also be eating ok but exercising too much (young athlete). This is something i would definitely talk to her doctor about. Something is going on.