What Caused Prolonged And Heavy Periods While On Birth Control?

I take birth control to stop my period because my menstrual cycles are painful/heavy. I've taken my birth control (desogen) for about 3 months than I got my period. The first week it was brown/bloody discharge, very light. Now I'm on week two and theres more blood, with some dark brown discharge as well. I took the anti-biotic Ciprofloxacn for 3 days. A week before my period started. How long is my period going to last? Do I need to see a doctor?