Okay, hang with me here...
What you describe is common with shoulder injuries due to the weakness and muscular substitutions that develop. When one muscle's function declines another usually tries to take over. Nice concept, but it can alter function with resultant pain.
If the infraspinatus is weak (remember the painful muscle on the back of your shoulder blade
) the levator scapula (shrug muscle under the traps) will get tight to take up the slack. This puts the rhomboids on constant tension. The dorsal scapular nerve is under the rhomboids and spinal accessory nerve is between the rhomboids and traps.
The constant tension can irritate the begeezus out of the nerves causing a constant achy feeling back there.
Treating the infraspinatus like we did should help. I wish you'd have mentioned this at the retreat. I'd have done a bit more to keep tension out of the rhomboids.
Strengthening the external rotators and other scapular stabilizers as we talked about should help quite a bit as will the postural correction exercise.
If it recurrs, let me know and we'll approach it another way.
Bill