Why am I not having my period?

I'm 16 I started my period when I was in 6th grade had it once didn't have it again till a while, From 6-10th grade (10th is what I'm in now) I've been loosing weight left and right. My period is not regular at all! The last time I had it was 4 months ago I went 10 months without having it once and then I had it and everything was normal, I get cramps, I get breakouts sometimes but my boobs have defiantly  not grown since the 6th grade I feel like some of it is because the weight loss. I'm not a virgin I've had sex with my BF of a year, I'm not on birth control but I know I'm not pregnant. I just don't get why I'm not having my period regularly!