Bursitis is an inflammation of the bursae. The bursae are small, fluid-filled sacs found near joints. They act as a cushion between moving parts in the joint to stop muscles, bones, and tendons from rubbing together.
When a shoulder bursa gets irritated, it becomes inflamed and grows in size. This means there is less space in the shoulder for muscles and tendons to move around, which can cause pain and a loss of movement in the shoulder.