Most shoulder injuries rotate your shoulder forward and inward, causing a hunched position. And the secondary muscles will also tighten (your neck, upper trap, chest and arm muscles) with back muscles of the shoulder weakening.
Here is the stretch streps:
-Using any water bottle, lay face up at the edge of bed, sofa, etc.
-Start with your arm at your side with your thumb pointing up.
-Bend your elbow 90 degrees, so your forearm is above your body.
-Slowly raise your elbow toward your shoulder until you feel uncomfortable, or your upper arm is even with your shoulder and keep it there.
-Then rotate your upper arm back until you reach discomfort again.
-Hold for 15 to 30 seconds.
Repeat this stretch as much as you want to relieve pain, and improve posture.