No period for 6+ months

So I am 20 years old. I also have a two year old daughter. Since I hav had her my periods have been all over the place. This is the 2nd time I have gone 6+ months with no period. I'm not pregnant. I gone to the doctor and all tests(urine and blood) are negative. The tested my thyroid too. I've had vaginal ultrasounds and they just don't know what's going on. Do any of you?! I'm afraid that I'm not going to be able to have kids if the problem doesn't get fixed. I'm on birth control to see if that helps. I also might be insulin resistant. Please Help!!!!