A painful raised bump on the arm may be related to boils, subcutaneous hematomas, and sebaceous cysts complicated by infection. 1. Boils: Folliculitis can be caused when a single hair follicle and its surrounding tissues are infected by bacteria and other pathogens, and boils can be formed after the fusion of multiple folliculitis, which will appear as a raised bump on the arm and may be accompanied by painful symptoms. 2. Subcutaneous hematoma: If the arm is hit or rubbed, the trauma may cause a subcutaneous hematoma, which is accompanied by pain and a painful, raised bump on the arm. 3. Sebaceous cyst with infection: If the opening of the sebaceous gland is blocked and the sebaceous secretion cannot be discharged, a sebaceous cyst may form, and a painful, raised bump may appear if there is a localized infection. It is recommended that patients with painful bumps on the arms go to the hospital in time to clarify the cause of the disease and standardize the treatment under the guidance of the doctor.