I Am On A Continual Birth Control To Prevent Periods,

I am on a continual birth control to prevent periods, I am 48, the reason for that is I need a bladder surgery trying to put it of so it will last my natural lifetime, however I have had breaktrhough bleeding every two weeks for 4-5 days for 5 months, what is going on... they finally scheduled me for an ultrasound this coming week