Bleeding after sex???

i’m in my 20s, and ever since i lost my virginity at age 16, i ALWAYS start bleeding a couple hours/a day after sex. Sometimes it’s just a little bit of spotting, other times a full blown period will start (which is crazy because i’m on the IUD and i don’t have periods) and will only last a day or two. I’m in a long term relationship and we have sex regularly, but a at least a little bit of blood always comes up within 24 hours.

Does anyone know what is causing this???

Also, we both get tested every 6 months and we’re both std free