Late period

My period has been late. I have sex with my boyfriend last Thursday without a condom and then with and my period was due the 28 which was the day we had sex. Could the sex cause my period to be late? I had just came back from a 2 week vacation the day before which was the 27th . But before the sex I didn't experience any cramps or anything. Then I took the plan b pill 3 or 4 days after the sex and I am worried . Can I be pregnant ? Or what do I do I am only 17 .