Unfortunately the reality is, if you are having sex, you run the risk of becoming pregnant. As you probably already know, condoms and birth control pills are not 100% effective. Don't panic yet about an absent period, sometimes just worrying that you may be pregnant can cause your period to be delayed or missed completely. If you don't get your next period when it is due, you will need to take a pregnancy test. If you take a test before that, it probably won't be accurate so there's not much point wasting the money on tests at this stage. However, if you can't wait that long, you can ask your doctor to do a blood test to determine if you are pregnant or not. I believe these tests can be taken after approx. 7 - 12 days after the intercourse took place.
Good luck