It's possible. How soon after the unprotected sex did you get your period? How long did it last? Was it lighter than usual? Or different? And how long ago did you have unprotected sex? If you had a real period the odds are you aren't pregnant. But if it was just spotting it could have been implantation. If it's been at least 14 or more days since the unprotected sex you could take a pregnancy test and find out that way.
Hope it helps,
Krys