What's going on??!

So this month my boyfriend and I have had unprotected sex quite a few times (everytime being when my chart says 3% chance of pregnancy. Not ttc but not totally opposed to the idea) my period has been there days late and it seems as if I just got it. It's very light but my cramps are worse than they have ever been in my life. I have never been in so much pain and I can't figure out what is going on.