Periods Start On Taking The Birth Control Pills, Appear During Breaks, Stop On Getting Off Them. Help?

I m 15 years old and I finished a course of Dianette Birth Control Pills in June 2012 and since then my periods have not come back unless if I start the Pill again, in which they appear during the breaks but as soon as I come off of the pill they don t come back. I ve haven t been on the pill for months now and my period still hasn t come back and I am worried. There is no possibility of me being pregnant at all and I was wondering if my weight was a factor (I m 99lbs and 5foot7inches). I know that is underweight but before I started the pill my periods were fine. The whole reason I went on the pill was because my periods were too heavy and lasted for a long time and there is a history of anaemia in my family. Please help!!!!