Chance of pregnancy

Okay firstly please ladies don't jump down my throat I know I am wrong and I shouldn't be especially having unprotected sex while not understanding fertility etc. I don't know why I have never understood it but I really want to learn it as it is very important. I have sex on Monday morning and I was on top and he blew in me I had no idea and it was my first time with this guy so I didn't know he can't control himself. I had 5 days for my fertile window to begin so what are the chances of pregnancy. I checked around it says the chances are 0% but how certain is that? Any help is much appreciated xx