This probem you have is very very typical in weight lifters. Look at your shoulders. Are they in a protracted position? If so, then your problem could very well lie in the fact that the head of your humerus has rotated to a dysfunctional position on your glenoid fossa. The only real stabiization for the shoulder is the labrum. A circle of cartilage surrounding the glenoid fossa. your chest muscles may be tight and they are attatched anteriorly to the humerous and pulling it forward. your back muscles such as your Teres Major and Latissimus dorsi may be weak and allowing the chest muscle to pull the humerus forward. So now yoru humerus is in a proracted position causing altered joint motions. Work on your back muscles preferably the ones that connect to the humerus and back and this should pull the humerus back into functional position.
Hope this helped.
BTW I am not diagnosing your problem this is just for infrmational purposes