Very heavy creamy/yellow discharge and period 4 days late?

Hi!

I never had unprotected sex, although my bf used withdrawall (and took the condom off after he took it out of me). He got a little bit of cum on my butt but we wiped it off. This was after my Glow stated "fertile" days (like 4 days after and i had an increase in bbt and no more eggwhite CM).

Fast forward 14 days, my period is 4 days late and i am getting heavy yellow discharge, so much that it feels like I am on my period. I took a pregnancy test 2 days ago and it came back négative. I have an appointment for Sept 28 (earliest I could book with my doctor) but in the meantime does this sound like an STI? Or pregnancy?? Help please :((