Why are my periods so irregular?

I first got my period when I was eleven (Jan. 2007) and they were normal. I waited 28-30 days, and I would menstruate. It would last on average about four days. However, when I was thirteen (Jan, 2009) things started changing. The shortest I would wait for a period is 33 days. I bleed or three or four days, stop, and bleed again off and on for four more days. In February of 2011, I had two periods, none in March, and one again on April 15. I had one in May, and I just got off my period for June. I am fifteen now, and will turn sixteen in October. I am a virgin. Can someone point me in the right direction as to why this happens?