Yeah, women need some body fat to sustain menstruation - it is a built-in mechanism that prevents pregnancies in women who do not have sufficient resources to nourish a fetus properly. Anorexia commonly results in the loss of a period. Being very athletic can be the culprit, too, as Emily explained.

Waelwulf: what is concerning that you would not see a doctor. It appears that you have an irrational fear of doctors. Plus, if you are sexually active, you should be having PAP smear done annually, at your age.