If memory serves, it might be that when you are on birth control you do not have actual true periods. Rather, you bleed a bit, but the lining does not build up and shed the same way that it does without birth control. That's how it is with my Nuvaring at least, I'm not sure about the pills. My "period" usually starts a couple or few days before I put the ring in again, and tapers off just before then.