Bursitis is the swelling and irritation of a bursa. A bursa is a fluid-filled sac that acts as a cushion between muscles, tendons, and bones.Bursitis is often a result of overuse.It can also be caused by a change in activity level, such as training for a marathon, or by beingoverweight. Other causes include trauma, rheumatoid arthritis, gout, or infection. Sometimes, the cause can't be found. Bursitis commonly occurs in the shoulder, knee, elbow, and hip. Other areas that may be affected include