Honestly was just wondering if any of you have ever experienced this.
It started after playing my heavy Purple heart accordion, for hours, following a full tune up. For weeks, I've noticed playing any accordion makes my left arm drop out of it's bellows job.
If the pain is next to your scapula, it could be a pinched nerve due to a vertebral disk problem in your neck. I have the same thing on the right side. It comes and goes; I have learned to manage it. When it acts up, I can use stretches (carefully!) and at home traction to treat it. I mention this because when it started for me, the pain started in the shoulder blade. If it gets real bad, the pain can go down my arm all the way to my hand. If your issue is due to a disk problem,take heart. It usually resolves on its own, given time.