Sudden (acute) injury. -- A sudden (acute) injury may occur from a fall on an outstretched arm, a direct blow to the shoulder, or abnormal twisting or bending of the shoulder. Pain may be sudden and severe. Bruising and swelling may develop soon after the injury. If nerves or blood vessels have been injured or pinched during the injury, the shoulder, arm, or hand may feel numb, tingly, weak, or cold, or may look pale or blue.

Overuse injuries -- You may not recall having a specific injury, especially if symptoms began gradually or during everyday activities. Overuse injuries occur when too much stress is placed on a joint or other tissue, often by overdoing an activity or through repetition of an activity.

Muscle tension or poor posture.

Pain that is coming from somewhere else in your body (referred shoulder pain).

Breakdown of the cartilage that protects and cushions the shoulder joints (osteoarthritis).

Calcium buildup in the tendons of the shoulder.

An irritated or pinched nerve or a herniated disc in the neck.

Infection in the skin (cellulitis), joint (infectious arthritis), bursa (septic bursitis), or bone (osteomyelitis).

Invasive cancer that has spread to the bones of the shoulder or spine.

Abuse. Any shoulder injury (especially a dislocated shoulder) that cannot be explained, does not match the explanation, or occurs repeatedly may be caused by abuse.