I hope not..

My period according to this app is 3 days late. I have been having unprotected sex for over 2 years and I haven't gotten pregnant, not even a scare. I came to the conclusion recently that I'm probably just not able to, (I haven't actually checked with a doctor, it's just a feeling I have) and now that I feel like I have goals/dreams to accomplish, I'm worried that I might actually be pregnant this time.. I'm actually stressing out a little bit. I haven't had any period symptoms besides the occasional side cramp.. any ideas what could be wrong with my insides? I appreciate the response/help.