I'd get Plan B if you are worried about the sex the day before. It wont help if you are pregnant for over a week, though. Wait a week or two and take a pregnancy test.
CHILL though. You are scared and worried and stressed, and that is only making things worse. You're being stressed will only make things worse. Periods do not come at exactly the same time every month, and stress only serves to make things much worse.
Good Luck :)