The inexplicable swelling of the right arm may be due to the following reasons: First, the presence of muscle damage in the upper limb caused by improper exertion, i.e. muscle strain, which may be accompanied by increased pain during activity and slightly reduced at rest. Second, venous thrombosis. Venous thrombosis leads to restriction of blood return, and the tissue distal to the blockage of blood vessels may be significantly swollen. Third, there is no local infection. Swelling may be accompanied by skin redness, heat and local pain. It is recommended to go to the hospital for physical examination and to improve the necessary tests to clarify the diagnosis.