Condoms. Condoms. Condoms.
Please. Wear. Condoms. Always.
Not only can it help prevent pregnancy, it is great practice to always wear a condom so you don't get anything sexually transmitted. If you think it can't happen to you, think again. There are thousands, millions! Of people out there with sexually transmitted diseases. They probably thought it couldn't happen to them too... I guess they were wrong.
Wear. Condoms. Always.
Ok, stepping off my soap box now: onto your question.
You should relax. Chances are you are not pregnant. You could be, but probably are not. Wait until you think your period is due and get a pregnancy test. Or you could always go to planned parenthood. They give tests for free. Wait until your period is due and/or late though.