whats wrong with my ball python???

This is just a wild guess, I've never seen it in snakes, but I have in lizards, a fecal burn. Where they drag their tails through crap, and it becomes stuck between the scales where the acids and bacteria start to inflame then infect the tissue.
I lost a Uromastyx to this about 14 years ago after the injury went septic.

I was also thinking maybe feces if the snake is close to shed and it got stuck between the layers of skin.

Also could be a burn or rot of some kind.

Either way, it needs looked at ASAP.

he was poped about 2 days ago and i got him 2 days ago. he hasnt pooped so idk if hes constipated.i personaly think its bruising from being poped but if it doesnt heal in the next couple of days im going to the vet.

I would create a dustless sterile environment asap (like now)
- change any loose substrate with a sheet paper type ( paper towels or news paper)
- remove any decorations, only leaving 2 hides
- change the water dish with something smaller, able to drink from but not soak in.

- maintain heat/humidity standards

Get to a vet asap, after last reply I'm guessing bad sexing.
Was he probed as well?

If that's a bruise, NEVER let the guy who did that pop one of your snakes again...that's horrible. It does look like an infection to me, but if he didn't have it before being popped, it could be a bruise, yes.
You should also tell the guy who did it that he bruised the snake, and show him the pics.

I was not aware that snakes could show visible bruising.

It's true, you really can learn something new every day!