The piriformis is notorious for wreaking havoc on the glutes. But if you're literally feeling the butt hurt, the piriformis probably isn't the origin of your pain. But it is a player in the dysfunctional movement spectrum.
When a "do-it-yourselfer" gets seriously butt hurt, he or she will try to remediate it with a lacrosse ball, softball, or whatever ball they can find to smash the area and relieve the pain. The problem? The lacrosse ball in the butt treatment isn't a permanent fix.
By placing a hard object right up against the notch in your butt where the pain seems to be stemming from (likely the sciatic nerve) you're essentially attempting to target a nerve with the lacrosse ball. And a nerve isn't something you're going to "break up" no matter how much you believe in smashing or rolling it. Doing this repeatedly can cause constant irritation. And if that's what's happening, then you most likely did it to yourself.
The Fix: Foam Rolling the Glutes
Your pain in the butt is most likely due to an instability issue in the lumbar spine or pelvis, and one of the key players to adding stabilization to this region is the gluteus maximus.
By targeting the whole glute with soft tissue work, you can increase the surface area of the SMR tool that's coming into contact with your backside. You'll also be directly working on the prime areas lacking in stability. Stick to the foam roller for butt rolling and skip the ball.