Ovulating and late for period?! Could I be pregnant?

Okay so me and my boyfriend had unprotected sex alot at the beach early august and then I had a full on period. Now in september I'm 3 days late and ovulating? So confused???. He's in the airforce and there is no possible way I would have gotten pregnant from my period until now.. could I have been pregnant on my last period and not have known it? But why am I ovulating