Like the above poster said. If you've had normal periods, you're not pregnant. It's not possible to have a "true" period while pregnant. If you don't want to be pregnant, use protection. It doesn't matter if it's 5 minutes long or 5 seconds long. It only takes one sperm to get pregnant.
If you don't want to get pregnant you need to use protection. Having unprotected sex is a sure way to get yourself pregnant. You may have lucked out this time, but may not the next time. If your periods were on time and seemed normal, then it's highly unlikely you are pregnant. If you want to you can take a home pregnancy test, though it's not really needed.