Me and my fiancé had unprotected sex Saturday and he did ejaculate inside of me but later on in the day ( hours later ) I got my period. Is there a possibility that I could still get pregnant?

We have been trying, but my period was 3 days late with a negative test. I wasn't expecting to get my period the same day lol.

I've done some research, but I was wondering if anyone could break it down for me simply.

Thanks in advance!