The common causes of hidden pain under the armpit are lymphadenitis, soft tissue injury of the armpit muscles and breast hyperplasia in women. 1. Lymphadenitis: pathogenic bacterial infections cause swollen lymph nodes in the armpit, resulting in pain. 2. Soft tissue injury of armpit muscles: recently engaged in heavy physical labor or physical exercise, resulting in strained armpit tissues and pain under the armpit. 3. Breast hyperplasia: women can also consider breast hyperplasia, the breast tissue is distributed in the lower part of the axillary caudal lobe, and the hyperplasia causes hidden pain under the armpit. There may also be other causes of pain under the armpit, such as skin infections, etc. It is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, and carry out standardized treatment under the guidance of the doctor.