What Might Cause Spotting And Bleeding After Taking Sprintec?

Hello. I don't know whats going on with my body. I was on the birth control Sprintec for over a year and then I noticed about 3 weeks ago that I was spotting after I week of starting over with a new birth control pack. After I noticed the spotting, I then stopped taking the pill. I did not stop spotting and so then I went to the doctor and told me to take 400 mg of Prometrium for ten days to stop the bleeding. Well on the 5th day, I noticed that the bleeding has not stopped and so I stop taking them earlier this week. I am so tired and frustrated about this. The doctor cant tell me anything.