The pain of the bone under the armpit may be caused by the following reasons: 1. The damage to the rib cage may occur here, in the lower part of the armpit, although there is a relatively thick soft tissue cover, but the vast majority is subcutaneous fat, if it suffers a direct violent blow, it may directly cause damage to the rib cage, such as the possibility of contusion, or even fracture, which leads to pain. 2, The pain may be caused by infection, such as acute septic sweat gland infection or systemic infection, which may lead to pain in the bone. 3, it may be caused by internal lesions in the chest cavity, such as severe pneumonia and lung tumors, which may cause pain in the bone. 4. If the pain occurs in the left armpit, it may also be caused by heart-related pathologies, such as acute myocardial infarction.