Irregular Ovulation

I stared my period when I was 12. I am currently 17. It has never came at the same time of month. Convinced something was wrong I asked for a pap smear in August 2010 even though I was still a virgin. But the results were normal. The thing that worries me is that my period is becoming very inconsistent. Iv missed 2 months in a row once plus its off and on. like some months I will have it and others I wont. not exactly i will then i wont... it just depends. I was on birth control for 3 months and my period was consistent. but then after i stopped it was off again. I am supposed to start birth control again after this period. I also don't feel as though my ovaries aren't switching back a and forth evenly. (I tend to have more cramping from my right side) Also when i do have my period I have been getting sick and in a lot of pain so I have been missing some school. I weigh between 105 and 108 pounds. When I skip periods I will sometimes still get symptoms of a period like breast tenderness exc. but other times I have nothing at all, no pain, and no sickness.

Whats wrong with me? I am terrified that when I am ready to have kids I will be infertile. :((