I think I know what I did. I had my arm out the window of my car. When the auto lock function kicked in it pinched the bottom of my arm and I jerked my are up. Felt a twink in my shoulder. Since then, when I flex my shoulder muscles, a deep pain builds and becomes real bad. It's deep in my shoulder and lasts for about ten to twenty seconds. At least I think it's a rotator cuff injury.
Yeah, the pain can be excruciating. Can you reach behind your back and touch the opposing shoulder blade?