Plan B has a 25% failure rate and it will certainly effect your next period. So yes, you might be pregnant and yes, this could be a side effect of the pill.
Take a home pregnancy test 14 days after unprotected sex (10 days if you use first morning urine and a good test)
Could you be pregnant?? ahhh yes, anytime you have unprotected sex, you could get pregnant...or get diseases.
Plan A (not having unprotected sex) works BETTER than Plan B.