A bump under the shoulder may be related to lipoma, sebaceous cysts, boils and other causes. 1. Lipoma: Abnormal hyperplasia of subcutaneous fat tissue, appearing in the shoulder area, may form a bump on the surface of the body. 2. Sebaceous cysts: The sebaceous glands in the shoulder area are blocked, causing sebaceous secretions to accumulate in the glands, which can form cysts. 3. Boils: A hair follicle in the shoulder area is infected by purulent bacteria such as Staphylococcus aureus, which can cause a localized inflammatory reaction and the appearance of a swollen bag. It is recommended that the person who grows a bag under the shoulder should go to the hospital in time, clarify the cause of the disease, and carry out standardized treatment under the guidance of the doctor, so as to avoid delaying the condition.