Sex on first day of period

Just had sex on my first day of my period. I am on the pill. Been on it for 3yrs and take it every day. ( I will be honest I’m more horny on my period then regular times ) my bf just made me really horny so I just went to the bathroom and took my tampon out and we had sex. He came in me and it’s my first day of my period. I know the pill prevents pregnancy from happening but I also know there is a slight possibility you could still get pregnant so I’m asking what’s the percentage of pregnancy for me at the moment.