beleary
"No Masse!"
You most likely have what is known as an impingement syndrome. As you elevate your arm, the rotator cuff tendon becomes pinched, or "impinged", between the bone in your upper arm (humerus) and the bony part at the top of your shoulder (acromian process). This leads to rotator cuff tendinitis and shoulder bursitis.
If you can assume a position where your arm is not elevated as much, it should help. Shooting from a higher position, or flexing your elbow a bit will allow you to lessen your arm elevation relative to the shoulder joint.
Yep. This is my problem. I got cortisone shots and most of the pain went away. Still have some problems with my upper bicep. I never thought about the weight I put on my arm while shooting. I'll give that a try.