Shoulder pain when lying on the side can be caused by cervical spondylosis, frozen shoulder, rotator cuff injury, myofasciitis of the back of the shoulder, trauma and other factors. 1. Cervical spondylosis: Neurogenic cervical spondylosis can lead to nerve compression and shoulder pain, and when lying on the side, the muscles and nerves are pulled, which can lead to aggravation of pain symptoms. 2. Frozen shoulder: In frozen shoulder, aseptic inflammation may occur in the shoulder joint capsule, surrounding ligaments, tendons and synovial membrane, which may compress the local tissues and aggravate the local pain and stimulation when lying on the side. 3. Rotator cuff injury: Repeated over-activity or trauma and other factors can cause rotator cuff injury, neck and shoulder pain, weakness and difficulty in movement, etc. When lying on the side, the local soft tissues are pulled and compressed, which can cause pain. 4. Shoulder dorsal myofasciitis: the aseptic inflammation of muscle fascia at the back of the shoulder can aggravate the tension and spasm of local soft tissues when lying on the side and cause pain. 5. Trauma: Trauma can cause bone and soft tissue injury of the shoulder, which can compress the injury place when lying on the side, resulting in pain symptoms. When lying on the side of the shoulder pain, you need to consult a doctor in a timely manner, make a clear diagnosis, and under the guidance of specialists, actively carry out the appropriate treatment, which will help to relieve the pain and other uncomfortable symptoms as soon as possible.