It may be a condition known as shoulder impingement. The shoulder joint is formed where the humerus meets the shoulder blade (the "ball" meeting the "socket"). There is a space where this connection is made, allowing several tendons and burase to lie here. Several factors, such as poor posture and excessive time at the computer can cause this space to narrow, putting pressure on these structures. This in turn leads to inflammation, pain, and weakening of the muscles that are meant to prevent this compression. These muscles, known collectively as the rotator cuff, are necessary to properly stabilize your shoulder as you reach overhead.
Symptoms of shoulder impingement may include pain when reaching behind your back or overhead for high shelves, as well as pain with carrying heavy objects and lying on your side.
In order to calm down the aggravated tissue, we need to open up the space in your shoulder. First, start by resting your arms on arm rests at your desk. When lying down at night, place a small towel roll or pillow under the arm that is causing you pain. Lastly, here is a simple exercise to start opening up the space in your shoulder: Scapular Retractions
Slowly pull back, activating the muscles in between your shoulder blades. Try to relax your neck muscles while performing this exercise. Hold for 3 seconds, perform two sets of 10.