I have this pain in my back under my shoulder blade. I got this from reaching all the way back and flinging my arm and stretching it out and towards home plate. It gives me a lot of velocity on my fastball but it’s sort of a tweak pain like I’m using my shoulder blade and triceps to pitch. I’m guessing these are poor mechanics, but the ball really jumps out of my hand.
This sounds like a problem decelerating your arm.
It could be that you have built up the muscles that aceelerate your arm but haven’t built up the muscles that decelerate your arm (e.g. Teres Minor). Your follow-through could also be causing your problems (e.g. stopping your shoulders from rotating).