There are many reasons for the hidden pain on the outside of the breast under the armpit, such as inflammation of lymph nodes, breast hyperplasia, breast cancer and so on. 1. Inflammation of lymph nodes: The lymph is rich near the armpit area, if the lymph nodes are inflamed, it can cause breast pain, so there will be a hidden pain in the outer side of the breast under the armpit. 2. Mammary hyperplasia: Mammary hyperplasia can lead to the expansion of breast ducts, which is easy to appear breast pain, accompanied by small nodules, especially in the pre-menstrual period, the pain is more obvious, and after the end of menstruation, there is relief, so there can be a hidden pain in the outer side of the armpit of the breast. 3. Breast Cancer: Hidden pain on the outer side of the breast may be caused by breast cancer, which is accompanied by cellulite-like skin changes and fuzzy boundaries of nodules, and can be diagnosed clearly by breast ultrasound and breast biopsy. Pneumonia, skin disease, intercostal nerve disease, and heart disease (coronary artery disease) should also be considered as possibilities on the left side. There are many reasons for the hidden pain in the outer side of the breast under the armpit, after the appearance of the symptoms, it is recommended to go to the hospital in time to improve the examination, clear diagnosis and take standardized treatment according to the doctor’s instructions, do not ignore the symptoms, and take it lightly.