What kind of birth control pill are you on? I have been on both
Tri-Spintec (generic
Ortho Tri-
Cyclen) + Ortho Try-Cyclen Lo, and I know that if you miss 2 or 3 days of the pills, it will give you an early period. Were you taking it consistently before those days that you missed? And did you have sex after those missed pills?
Either way, I think that it is probably because you missed those two pills, but if you want to know for sure go pick up a pregnancy test.