had sex and I had JUST started my period; still big possibility of pregnancy?

Me and my boyfriend had sex today, we were in the moment and he ended up just going inside of me… after we got done having sex I went to the bathroom, and I had started my period. How much of a possibility is it that I could still get pregnant since technically I don't really know if I was on my period when we had sex?… right before we had sex I went to the restroom and I hadn't started yet, but after sex when I wiped there was a little blood.