Osteoarthritis, when the cartilage in your joints thins and causes wear, can cause shoulder pain as your joints will be unable to move as fluidly as before you had the condition. Although the joints do repair themselves, this can often have side effects such as swelling when these repairs change the shape of your joints.
Shoulder pain can also be caused by conditions such as polymyalgia rheumatica, whose symptoms are pain and stiffness in the muscles, including the shoulders. If you have this condition, pain may coincide with other symptoms similar to flu, such as a fever, and the pain will not be isolated to your shoulders. The pain can be found in other areas of your body, such as your neck and hips. You will have pain in both shoulders usually, and they may also be inflamed.
Rheumatoid arthritis can also cause pain to your shoulder joints, and you may also get stiffness and inflammation in affected areas. When you have rheumatoid arthritis, your immune system attacks healthy tissues, which can cause wear and even disintegrate your joints, leaving your shoulders in pain.