Pain in the armpit and arm at the same time may be caused by the following diseases: 1. The most common orthopedic disease that causes pain in the armpit and arm at the same time is periarthritis of the shoulder joint, also called fifty shoulder, which occurs mostly in middle-aged and elderly people over 50 years old. In particular, the inflammation of the back of the shoulder can cause pain in the armpit, and in severe cases, the pain radiates to the arm and the patient does not dare to touch the back with the palm of his hand, and the pain becomes more intense the further up the shoulder is touched. Some patients with atypical angina may also have pain in the axilla and arm, but the symptoms do not last long and the shoulder joint movement is not restricted, even when the pain is present.