What Are the Most Likely Causes of Armpit Pain?

3. Swollen Lymph Nodes

Another cause of armpit pain is a swollen lymph node, which indicates that something is causing a response from the immune system. They may or may not be tender. A swollen lymph node can be as big as a pea, golf ball, or larger, depending on the cause. In most cases, swollen lymph nodes are a normal reaction to an infection, though when found under the armpit, there is a possibility they are a sign of breast cancer and should be checked by a doctor.