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I agree completely with the above answer but would ad that you should take a home pregnancy test today and again In a few days. If these are negative you can take comfort in this. If positive you should contact your OBGYN and stop the pills immediately. It is highly unlikely that you got pregnant from the most recent encounter but people CAN get pregnant on birth control pills so the possibility of a pregnancy from a prior encounter, even with pills taken correctly, is there, but remote.
If there is no more bleeding and no more pain and the tests are negative you can simply wait for your next period.
Any acute pain in the abdomen, especially if accompanied by bleeding, should be evaluated immediately as this could represent an ectopic pregnancy, HIGHLY UNLIKELY AND NEARLY IMPOSSIBLE WITH NEGATIVE HOME PREGNANCY TESTS, but theoretically possible.
Odds are WAY in your favor that this is simply breakthrough bleeding as described above, but adding a couple of home pregnancy tests will go a long way to making sure this is all that is going on.
I am standing by if you have further questions for me.