Go to the drugstore and get a home pregnancy kit.There are lots of them. Depending on how many weeks it has been , they should be pretty effective about telling you whether or not you are pregnant. ( read the instructions on the test to make sure you are at an appropriate time for a reliable reading ) If you want to be totally sure, go see a doctor and he or she will examine you and give you a diagnosis that will be extremely reliable.
If pregnancy isn't something you wanted, you could try the Plan B, but I think you have to take that within 72 hours of unprotected sex.
A pregnancy test would be your next option as far as a determination. It's probably too soon for an over the counter HPT. Not sure if a blood test would show anything this soon or not. You may want to contact your doctor and see what kind of advice they can give you.