The most common reason for women to find small bumps under the armpit while breastfeeding is paronychia, which is mainly painful periodically, and for women who have returned to normal menstruation during breastfeeding, the bumps are usually painful before menstruation and are granular and painful to the touch.
If a black dot is found in the center of the bumps and the bumps are elevated in the skin, then it is considered a sebaceous cyst, also known as a “pink tumor”, and sebaceous cysts can be painful if secondary infection occurs.
If the bumps are hard, less mobile, and fused with each other, and if they are accompanied by breast tissue invagination with an “orange peel appearance,” it may be a metastasis to the axillary lymph nodes of breast cancer.