Pain under the armpit consider muscle injury, lymphadenitis, paronychia, folliculitis and other causes. 1. Muscle injury: for example, injury to subscapularis muscle due to external force, mainly for the pain of internal retraction and internal rotation. 2. Lymphadenitis: nodules can be felt under the armpits with pressure pain, which can be clearly diagnosed by ultrasound. 3. Paramastia: localized swelling and pain during menstruation, and a soft mass can be felt. The color ultrasound can make a clear diagnosis. 4. Folliculitis: localized redness, swelling and pain, serious pus infection. If the pain under the armpit is obvious and is not alleviated by rest, you need to go to the hospital for further examination to make a clear diagnosis and further treatment.