Depo and pregnancy?

I had my first and last depo shot back in March of this year, I bled nonstop for four months and then, in August I had my first period since the depo shot, it started on the 2nd and lasted until the 26th. My period the next month ((September)) started on the 2nd and lasted until about the 20th of this month ((October))!!! There have been many times that my boyfriend and I have had unprotected sex and he has cum inside of me. Yet I can't seem to get pregnant. Am I losing my mind or is there a chance that I'm not fertile?! =/ any and all comments are welcome:)