Birth Control Pill Questions/Problems?

I started taking my first pack of 21-day Yasmin birth control pill on December 22, 2008, when my period started, as instructed by my doctor. After about a week of taking the pills, my period continued for the whole 21 days while I was taking the pill. It started to ease off when I stopped for my 7 day break, but then it started to pick up again. Now I've been on my period for close to 4 weeks. This is my first time taking the pill, and I'm only 15. I am not ually active, I'm just taking the pill to prevent myself from having another ovarian cyst. Is this something that could happen, or should I go talk to my doctor?