The pain in the right arm may be caused by pressure and cold, muscle strain and osteoarthritis, etc. The pain in the right arm may be caused by pressure and cold. 1. Pressure and cold: If the patient sleeps with his right arm pressed down at night, resulting in poor blood flow, or does not cover the right arm with a good blanket, resulting in the right arm catching a cold, the right arm can not be straight and especially painful symptoms may occur. 2. Muscle strain: if the patient is engaged in heavy physical labor or poor posture and other reasons may lead to muscle strain in the right arm, which may lead to the phenomenon that the right arm is not straight and especially painful. 3. Osteoarthritis: Osteoarthritis can be triggered by trauma and osteoporosis. And such patients are mostly characterized by pain, stiffness and impaired mobility of the diseased joints, so the phenomenon of not being able to straighten the right arm with special pain may also occur. To summarize, there are many reasons for the pain in the right arm, so it is recommended to go to the hospital for a detailed examination to clarify the cause of the disease, and then treat it accordingly.