Bursitis is inflammation of a bursa or bursae (more than one bursa), small fluid-filled sacs that cushion areas of friction around joints. Bursae contain synovial fluid that lubricates the joints. Bursitis typically occurs as a result of overuse during physical activities or infection of the synovial fluid. If a bursa becomes infected or irritated from repetitive stress, it will cause pain and limited movement. Bursitis is most common in the shoulder, knee, hip, elbow, or heel.