Suggest Treatment For Irregular Periods

Hi, I am a 28 yr old, female, 123lbs and I have always had an irregular menstal cycle. I first recieved my menstral cycle at the age of 11 and second menstral cycle at 13 years of age. I have lapse between each cycle anywhere from 6- 15 weeks. I have tried to regulate with birth control, but I have pms symptoms from them all. I am active, but my body fat level is not less than 17%, why is this so?