I had this in my left shoulder, and remember the pain when I slid on a water spill in our kitchen and extended my left arm to balance myself. It was extremely painful, even though my arm didn't contact anything. The pain seemed to be in the muscle between the shoulder and elbow when I moved my arm experimentally, but it felt like it was coming from the shoulder. It had an odd cure - I had to lop back my bay tree and spent the morning carefully chopping branches. I had a shower and found that my left hand could reach my upper back again. I've had no trouble since. I'm not suggesting heavy gardening as a cure! But in my case it seemed to unstick something.