What Causes Loss Of Menstrual Cycle?

I am 31 y/o female.Haven't had my period in 5 years.Had every possible test done( hormones, ppituitary gland)etc...they found nothing wrong.Had suffered from eating disorder in the past but not for the past 3 years.I am normal weight now.3 years ago dr put me on birth control pills so i would have induced periods and after 2 months took me off them to see if periods would come back without the pill but they didn't.My bone mass is down to minus 2.2 which very osteopenic for someone my age.What could be wrong?