A woman can get pregnant at any point in her cycle. According our handy Pregnancy Risk Assessment Flow Chart, there is a moderate to high risk that your a woman may get pregnant having unprotected penis-in-vagina intercourse.
We don't have some magic way of telling over the internet whether she is pregnant or not, though. Your girlfriend needs to get tested, pronto. In many countries, home pregnancy tests are readily available at most drugstores. If it tests positive, she should get to a doctor, pronto, to have the results verified. If it tests negative, repeat the test in 2 weeks just to be sure.
Furthermore, you are now both at risk for a variety of nasty sexually transmitted diseases, and you should BOTH be tested.
And in the future, always, ALWAYS use a condom and lubricant. Being 16 is stressful enough without worries like this. We have some great articles on this site that can help you with advice and information, so check 'em out.