If you've ever tried doing high face pulls, for rear delts, with your arms raised up then you're going to want to see this.
A common cause of pain in the back of the shoulder is from two rotator cuff muscles in the back of the shoulder - the infraspinatus and teres minor. Out of the two, the teres minor is easier to injure, especially in exercises that require rotating our arm backwards (external rotation). In sports it's most common in throwing movements. It's the "pull back" or cocking phase of the throw that can cause an issue with your teres minor. You would feel this pain low in the back of the shoulder. In the gym you're vulnerable to shoulder injury in movements like the snatch (powerlifting movement) or in high face pulls with your arms up.
My good friend, and Sports Physician Dr. Baron gives some advice on how to diagnose the pain in your shoulder, identify some of the causes, and then go over some possible solutions to alleviate the pain and help it heal faster, so you can get back back to training hard.