Have you been taking your pills regularly every day? Have you missed any pills lately, experienced vomiting or diarrhea, or taken any contradicting medications? All of these instances CAN cause the pill to be less effective and/or cause you to experience break-through bleeding.
If the answer to all of these questions is no, then your chances or getting pregnant are VERY slim (under 1%).
Your body could just be reacting to the hormones in the pills in a weird way, which happens all the time. Or, your symptoms could be that of a more complicated condition.
If your abnormal bleeding patters continue, I would suggest seeing a doctor.
I doubt that you are pregnant, but if you'd like, take a test.