What Causes Absence Of Periods?

I am 28 years old. have 4 kids. Started my period at age . It has always been very normal till about 2 years ago. I started having a period every 3 months thatly lasted 3 days. I went to my obgyn and she said as long as I was having a period every 3 months it was ok. and she gave me some pill to make me start. Well- its been 4 months and I have started. Preg test are negative. I burn up on the inside a lot. Could I be having early menapose _spelling- or ovarian cancer?