My doctor has me on a multi-month birth control plan. I originally took the active pills for 4 months and then the placebo pills for a week. I am now on a different, but similar plan with 3 months of active pills and 1 week of placebo pills. This should be my second cycle of the 3 month plan, but I accidentally took an extra month(making the total count of active pills 4 months) and haven't had a problem until this week. I have had heavy bleeding that is dark brown to black for the past 3 days. And today it has turned to a bright red. I am on the last week of active pills. I have also been taking an antibiotic the past 8 days for a sinus and urinary infection. I have heard it mentioned that antibiotics mess with birth control pills causing accidental pregnancies, but never breakthrough bleeding.

Any idea on why I'm having breakthrough bleeding?