HI!
If you were my patient, I would advise a pregnancy test--that is certainly one possibility, and it would be prudent to know whether the test was negative or positive! Your symptoms are common, though, and If you were my patient AND your test was negative, I would recommend starting your new pack 7 days after you finished the old one. (I am assuming that you have a 28 day pack).
One option for this cycle, might be to use emergency contraception such as Plan - B or Next Choice.
Good luck!
Dr B
I don't have any experience with the pill, but it sounds like it could be implantation bleeding if it is indeed pregnancy. Take a home pregnancy test with your morning pee and see where that takes you. If anything your period will show up soon