Pain under armpits. Lymph nodes not enlarged, blood work is all normal. Dr thinks its muscle. Its been over a month. Less in the evening, more daytime?

Armpit Pain. Pain can occur under your arm for a number of reasons. One of the most common reasons is a brachial plexus injury. This is trauma that occurs when the shoulder is pressed down and the head is pulled away from that shoulder. It often occurs in contact sports or from a fall. In addition, problems with the nerves of the arm, or brachial plexus, such as neuropathy or dysfunction, can cause pain.