I had same problem with right shoulder (dislocated and blew out rotator cuff back in college). Everything was fine when I was teaching yoga, but had to stop due to lack of time. Desk job leaning on elbows using mouse and keyboard all day ended up giving me tight pecs. Shoulder started to naturally dislocate itself forward, would pop back in place after rotating shoulders back. Due to neglect, I ended up with hypermobile upper back and frozen shoulder.

Now working my way back to recovery. Currently pain relief is coming from pills (with food), heat/cold, and lying on a rolled up towel in the center of my back. (It stretches by pecs and pulls the scapulas together.) Also alternating shoulder shrugs, pec stretches, rowing and bent over shrugs to pull scapulas in place to develop stability for longer term relief.

Good luck. I feel your pain. Hope that helps.