This is exactly where I had bad pain for over a year on both sides and where I now have atrophy. I thought for a while it could be supraspinatus, but it is the middle trap in my case. They attach to the scapula in the same place.
Supraspinatus works on arm abduction (raising arm to the side) and middle trap does scapular stabilization and retraction when reaching backwards.
If any of these movements increase the pain, it might be the clue.