The hard lumps under the skin of the arm may be caused by trauma, sebaceous cysts, dermatofibroma and other reasons. 1. Trauma: when the patient’s arm is hit by an external force triggered by arm trauma, may lead to the arm of the internal muscle wall hematoma or edema, which will appear in the pressure pain, blue and so on. 2. Sebaceous cysts: Sebaceous cysts are rare benign skin tumors with sebaceous gland differentiation, most of them are located below the skin close to the dermis, with hard texture, and there will be pain and bruising when pressed hard. 3. Dermatofibroma: Dermatofibroma is a kind of benign tumor of skin, mostly located in the dermis and subcutaneous tissues of the skin, the swelling is relatively hard, and the pain is obvious when pressed, and some of them may be accompanied by itching, and seasonal episodes. It is recommended that the patient timely examination, clear causes for targeted treatment, pay attention to skin care in life, monitor the development of the disease, and follow the doctor’s instructions for regular review.