I am on a diet and not getting regular monthly period. What to do?

Hello doctor,

Over the last several months, I have been dieting to lose weight that I gained last summer. Though my weight has fluctuated over the months, I have lost approximately 10 lbs. I am also an athlete, and thus I exercise daily. After the first two months of dieting, I noticed that I did not have my monthly period. It is not unusual for me, as my menstrual cycle has never been regular since I started at age 15. I am worried primarily about the health of my bones, but I do not want to stop dieting or gain weight. Also, I cannot decrease my exercise since it is a part of my schooling. I am also worried about going on birth control because of the numerous side effects associated with it such as breast enlargement and weight gain. Moreover, I feel as though this would only further interrupt my menstrual cycle. I feel that there must be some other treatment I can undergo to either stimulate or approximate my menstrual cycle more naturally. Thank you for your advice.