Read your post and thought I have got virtually the same symptoms. Last year I had an issue with my left shoulder and this turned out, after a thorough exam by my GP followed up by an ultra sound at the local hospital, to be a case of impingement syndrome where a piece of cartilage get trapped. Physiotherapy resolved the issue.
Fast forward to September this year and I have very similar symptoms to yourself but in my right shoulder. The soreness starts in the lower right side of the back of the skull and radiates into the shoulder and on into the back. Certain movements, like taking off a jacket, are agony and yet other movements like using the arm to carry heavy shopping cause no pain at all. In short, it's only if the joint is moved in certain planes that the excruciating pain occurs.
I have just had a raft of scans including a bone scan not because of this but because my PSA is rising again after EBRT so I'll know soon enough. My gut reaction is the pain is entirely dependent on which way I move. I suspect a met type pain would be constant?
Anyway, hope it's nothing too ominous. I suspect it's something unrelated but I get the "OMG it's spread to my bones" thought exactly.