You didn't pay attention in health class. Yes, it is highly possible that she is pregnant. She should take a pregnancy test. If you don't want to be a father, don't have unprotected sex.
well, its not that hard to go to a chemist and buy a kit.
I did the other week for my girlfriend, cos she hadn't had hers, and about 2 hours after I spent the 7 quid on it her period came on anyway.
Good luck, hope the chemist doesn't give you all this "I hope you get the result your hoping for anyway " crap on you