What are the chances I'll get pregnant?

I had today. We did not use a condom, and he d inside of me. I ended my period around the 24th or 25th. Today is the 29th. However, seldom does my period ever come every month. It didn't come all of June or July, and came only at the end of August. I'm only 15 years old. I know for certain it is a possibility to become pregnant, but my boyfriend and I are quite nervous. Any ideas as to what the chances are this will actually happen, will be helpful and greatly appreciated. Thank you.