Welcome to EmpowHER. Anytime you have unprotected sex you are at risk of an unwanted pregnancy. It being your first time, you should use some method of birth control. Like condoms or birth control pills. Emergency pills, should not be a method, for every time you have unprotected sex. Emergency pill can cause some side effects like changes in your next period. If your period was due, it's unlikely you were ovulating. You should wait a few more days and if still concerned, take a home pregnancy test.
Here are some resources for you