If the doctor is worried about you being too thin and you are continueing to purposefully lose weight and not do what it takes to maintain where you are at, I think there is reason for concern. Have they tested where your bmi is, if that is way lower than normal, it would make sense that you are not menstuating, and should definitely get that number up a little bit. Have you always been really regular in that area. Also, the hair falling out sounds like it could be from vitamin deficiency. Have you tried taking vitamin suppliments, that might help with that. WHat does your doctor say about the hair loss?