What Causes Abnormal Vaginal Bleeding While On Birth Control Pills?

I started having vaginal bleeding yesterday morning and its been coming and going ( not constant) and it looks differant than regular menstrual blood. I also take progesterone only birth control pills and hadn't had a period for atleast 7 months. Once in a while I get this really bad pain in my abdomin kind of like cramps. When I use to have a period regularly before bc I rarley ever got cramps though. What do you think is happening?