So, a few years back, I dislocated my shoulder, and it wasn't relocated before it was put in a sling. Now, if i strum fast for a long period of time, my shoulder starts to hurt, and my arm cramps up. This is mainly on acoustic. Now, I've tried many different positions and I've loosened my body up, but nothing is working. I started taking some medicine for my joints, but I'd like to see what you guys think. Any suggestions?