Hello,
I weight lift myself, and I find that a common problem in gym goers is that they train too hard and too heavy on the bench. Over a period of time this enhances muscle strength, but in doing so the Pec Major begins to shorten in terms of sarcomere length (Ref) as it 'pulls' on the arm rotating the humerus inward, while creating a stretch on the opposing back muscles.
The best way to correct this is through muscular balance exercises to begin with, as this is much preferable over surgery. As well a specific stretching program is recommended.
I would suggest you see a Physiotherapist so that you can have your posture analysed, and the specific muscles can be determined that require either stretching or strengthening.
Ref:
J Anat. 1978 December; 127(Pt 3): 459–468.
Changes in sarcomere length and physiological properties in immobilized muscle.
P E Williams and G Goldspink