Lipoma, sebaceous cysts or enlarged lymph nodes may be the cause of the bumps under the skin of the arms, and you should ask your doctor to find out the cause of the lumps in time. 1. Lipoma: the cause of lipoma is still unclear, it may be related to heredity or abnormal fat metabolism. Typical manifestation of lipoma is subcutaneous swelling, with clear boundary, tough texture, no pain and other symptoms, and the cause can be clarified through ultrasound examination. 2. Sebaceous cysts: Sebaceous cysts are formed when the sebaceous ducts are blocked due to exuberant secretion of sebaceous glands. Sebaceous cysts can be as small as a bean or as large as a small orange. If the sebaceous cysts are large or interfere with your life, you can consider surgery to remove them. 3. Enlarged lymph nodes: Enlarged lymph nodes caused by inflammation, tumor, infection and other factors can also lead to the above situation. You should actively search for the cause of the disease and then treat the cause of swollen lymph nodes. If you feel unwell, you should go to the hospital in time to avoid delaying your condition.