When I taught Pilates a number of my clients ended up with shoulder problems because of the hunched over (rounded shoulder) position they used in Spinning classes. Many people who don't "spin" have rounded shoulders from sitting over computers, and poor posture.

In this position the chest muscles become overly tight and the back muscles become weak and overly stretched. So, in addition to stretching the pecs, strengthening the upper back muscles is necessary.

In addition to the chest, the shoulder is affected because when the shoulder is turned inwards, the muscles at the front of the shoulder tend to become tighter that normal and the muscles at the back of the shoulder tend to become weaker than normal. Strengthening the rotator cuff can help to support better posture at the shoulder.

Baba, What MrsD wrote about frozen shoulder is Very Important. Many people who have shoulder pain stop using the joint normally and frozen shoulder can develop. If you do not work to keep full range of motion it can be difficult or even impossible to restore full range of motion.