Shoulder tendonitis is caused when the tendons in the shoulder are torn or stretched when the arm is used in an overhead position during such activities as swimming or tennis, claims the American Academy of Orthopaedic Surgeons. People who lift things over their heads for a living also suffer shoulder tendonitis.
Shoulder tendonitis is also caused by injury or by no discernible reason, according to the AAOS. In the early stages, the symptoms of shoulder tendonitis might be so minor that the patient chooses not to see a doctor. The pain, though mild, might be felt both when the arm is active and when it's at rest. The pain radiates down the shoulder into the arm and comes suddenly when the person tries to lift or reach with the affected arm. The pain also comes when the person tries to throw a ball overhead or serve or lob a tennis ball.
If the injury is not treated, the person feels pain at night, and the affected arm weakens and loses its range of motion, notes the AAOS. The person also feels pain when she tries to use her arm behind her back. The pain may be sudden and severe, and the shoulder becomes very tender.