Bleeding during sex

So I got check for Sti's last week and I'm clean. So no std's for sure and I did all the test and it says I'm clean. I just want to know is it normal to bleed during sex. If I haven't had sex in a few months it happens. I know it's not happening because of an Sti as I got checked so it's nothing to do with that. I just want to find out what it could be? I had sex last night and bled a bit. I am due my period in two days and the last time i had sex was 3 months ago but still this has happened before.