This happened to me also when I was about your age...
I had a lot of off periods between age 14-18...three months apart, sometimes longer etc. I was not having sex so I did not worry about it overly much. Then, when I was 18 I was working in this restaurant and this man and woman (older) I used to wait on, a favorite couple of mine, invited me over to their house to look at..pictures? (looking back, guess it was not the real reason)? Don't remember exactly why I went over there, only that they invited me. What I DO remember is after some small talk the man got me to lie face down on the floor or table or something and told me he was a chiropractor....must have driven him crazy to see my misaligned shoulders every week serving him food. Anyway, he ended up adjusting my back and told me, quietly (even though I never mentioned this to him) 'this will clear up your female problems' and darned if it didn't! So it is possible your spine is very misaligned and a simple adjustment or two by a good chiropractic doctor will fix your ticket.