There are several common causes of arm and armpit pain. 1. One of the most common causes is a strain of the surrounding tendons. A strain injury to the tendon caused by violence or chronic activity can cause pain in the arm and armpit. 2. Infection of the surrounding tissues, especially deep tissue infection. Infection can cause edema in the shoulder and armpit, which can promote pain. 3. Arthritis can stimulate the joint capsule, causing edema in the surrounding tissues and causing pain in the arm and armpit.