Shoulder bursitis occurs when the bursa becomes damaged, irritated, or inflamed. The bursa is a fluid-filled sac on the side of the shoulder that cushions the rotator cuff tendon. Certain positions, motions, or diseases can cause friction or stress on the bursa. When the bursa becomes injured, and the tendon does not glide smoothly, bursitis and pain can be a result of that. Shoulder bursitis can be caused by repetitive motions (overhead reaching or lifting, throwing, or twisting of the arm), muscle weakness or poor muscle coordination, incorrect posture and/or direct trauma (being hit, or falling on, the side of the shoulder).