To me, sounds like a typical (looking for the wrong thing syndrome )…
Sounds like the shoulder girdle is out of whack = the scapular is not in position.
This causes issues esp when under load.
Now, if the scapular is being Pulled out of place, from where?
It sounds like (without feeling it) that the biceps, the pec minor, maybe forearm, deltoid all have nasty trigger points. As this has been going on for over 2* months, I would check for compensation issues too with the other larger muscles around them areas.
1st, look after the bigger stronger muscles, then as they get better, smaller muscles. It’s like a tug of war, the bigger, stronger muscles will overpower the stabilizers.
Depending on how good a therapist you find, you could be fixed by the weekend, at least 80-90% fixed.