So I had my rear bumper replaced due to someone backing into me and ripped it off but when I got it back from the body shop it's not lining up and they said it's an OEM bumper. So why isn't it fitting?can someone help me o m

 

Crappy body shop.

 

Maybe your body is bent and the new bumper makes that apparent? I mean, your car was hit, so the damage might not have been limited to sheering a few a bumper bolts off?
A force strong enough to knock a bumper off surely could be in question of causing other damage.