How likely is it that a 17 year old girl has developed ovarian cysts?

It is not Unlikely for a 17 year old girl to develop cysts on her ovaries. You really would need to see your gynecologist to verify this. He/She may send you in for an ultrasound. There is actually a condition called Poly-Cystic Ovarian Syndrome which with your irregular menstrual cycles may be a possibility. I have PC-OS and once diagnosed was placed on a diabetes medication called glucophage (generic name metformin) which not only helped greatly with the cysts but also regulated my menstrual cycle so that it was normal flow and for the normal amount of days. Prior to that I would have very heavy flow for close to a month, severe cramping and all the other fun things that come with it. Basically....talk to your gyne. about your concerns and if he/she doesn't check them out...switch to a new one who is willing to do some investigative work.