Why are my periods so irregular?

I'm 16 years old and, as of the beginning of January, I will have had my period for 3 or 4 years - I cannot remember exactly how many it has been. I do a lot of sport and my weight is constantly varying. At the moment, my weight goes from about 8st10 to about 9st3. I am studying for my AS Levels, which does put me under quite a bit of stress. I have been in a relationship with my boyfriend for just over a year and 3 months and have been sexually active for 5 months. I am not on the pill - I cannot because I get migraines with aura - but I always make sure a condom is used, and we're careful about it. My periods themselves can be anywhere between 2 and 13 weeks apart and quite frankly it's annoying and worries me at times. My question is what is it that makes my periods so irregular?