I take my birth control pill everyday. i've had 4 days of breakthrough bleeding this cycle and stomach pain. how likely is it that i'm pregnant since?

Rather unlikely: If you take your birth control at the same time everyday and never miss doses the failure rate for oral combined birth control is as low as 0.3%(as in 3 pregnancies occur in out of every 1000 bouts of intercourse). Otherwise, if your use is sub "perfect" then the failure rates are reported in the 9% range. Based on what you report the likelihood is rather low.