Bleeding after sex (it’s weirder than it sounds)

So I know bleeding after sex is normal (especially if you’re doin it a little too hard 🥱😬) but the thing is I only bleed when my partner finishes inside of me. I have nexplanon so I know my body is doing its thing, but does it sound like I’m allergic to his c*m? Or does it sound like something else entirely? My cramps are pretty bad when I do bleed. Sometimes he starts my period from this happening too....I’m kinda concerned and just need some reassurance about what could be going on 😅 thanks ladies