Is there a reason that, when I am not on birth control, I do not have a period?!

You're not as underweight as you probably think you are. I'm 17, 5'2 and a half and 90 pounds and interestingly enough on a very similar birth control to you, the generic to yasmin. My body mass index is a little lower than yours, and I'm still gradually losing weight but my doctors have told me I'm fine. The rules for being underweight are subjective, they're guidelines. So don't freak out about that too much. And when you're on a pill you're not actually having a menstrual period, you're having withdrawal bleeding. So it could be that your body reacts to the withdrawal bleed but isn't functioning correctly to go through a proper menstrual cycle.

This explains the difference.

http://www.scarleteen.com/article/advice…

I'm still only speculating based off of a lot of research that I've done on this subject. I made sure I knew EVERYTHING about birth control before I got on it a couple years ago and I still take the same approach to sex in general because I'm very, very paranoid about getting pregnant. But I'm still not a doctor and this is unusual so you should see one. For all I know maybe it is because you're underweight, but I know for me it hasn't caused any problems.

It is most likely because you are underweight. And don't listen to the first person - you are quite underweight. The cysts may also have something to do with it. I suggest visiting your GP.