Possible causes of a painful bag on the shoulder include sebaceous cyst, tendon cyst or fibroma. 1. Sebaceous cysts: hair follicles are blocked by sweat or surrounding tissues and become inflamed causing sebaceous cysts, which can be red and swollen lumps and can be treated with erythromycin ointment. 2. Tendon sheath cysts: due to the chronic friction of tendons and tendon sheaths, fluid accumulation occurs, resulting in tendon sheaths bulging, and painful lumps can also occur. 3. Fibroma: caused by subcutaneous and submucosal fibroblast proliferation, generally the surface of the skin color is normal lump clear boundaries can be pushed, larger lumps and pain can occur. There may be other reasons for the pain on the shoulder, such as acromioclavicular arthritis, etc. You should go to the regular hospital in time, and the doctor will give you the correct judgment and targeted treatment according to the specific condition.