As the bench restricts the movement of the scapula, at the shoulder joint the ‘ball’ is forced to move to the end of its range in the ‘socket’.
Most likely, one of these three shoulder problems is responsible for your pain:
Improper muscle balance. If the strength ratio between two muscle groups is off kilter, you can actually experience faulty alignment. For example, if the strength of your pecs is far greater than that of the external rotators of the humerus (teres minor and infraspinatus), you’ll likely feel a sharp pain in the superior anterior portion of the upper arm (this problem is often misdiagnosed as bicipital tendonitis).
Adhesion buildup. One of the regrettable side effects of years and years of weight training is the buildup of adhesions in soft tissues and structures. Adhesions are a result of the load used and the total volume of repetitions. In other words, the more sets and reps you perform and the stronger you’ve become, the more adhesions you’ve developed. These connective tissue buildups can take place within the muscle, between muscle groups, or between the nerve and the muscle. Adhesions can occur in any muscle structure, but the one most often responsible for bench-press-induced shoulder pain is the subscapularis muscle. The good news is that they can be found and “cured” quickly through a soft tissue management technique called Active Release Techniques.
Lack of flexibility. Failure to stretch the muscles on a regular basis can precipitate the onset of injuries. You don’t need to become a Grand Master of yoga,though. Regular PNF stretching of the shoulder girdle before your upper body workouts will do wonders for keeping your shoulders healthy and functional.