This is a swelling of a fluid-filled sac called the "subacromial bursa." It's in the shoulder, between a bony protrusion called the "acromion" and the rotator cuff. You have similar sacs near other large joints throughout your body. They act as cushions between your bones and your soft tissue. Normally they have a small amount of fluid inside them. But sometimes they can swell. We call that "bursitis."
Shoulder bursitis is usually caused by constant stress or friction against your bursa. It can happen if you do a lot of repeated arm motions, especially with your arm raised. A lot of lifting and pulling can cause it. This type of bursitis is often a problem for painters and for construction workers.
Symptoms include pain and tenderness. It may be hard for you to move your shoulder. You may not have your full range of motion. You may feel pain during activity and when you are at rest. It can wake you up at night.
Treatment options include rest, medications and physical therapy. If these aren't helpful, you may benefit from surgery. Your healthcare provider can create a plan that's right for you.