A bag in the armpit is commonly caused by the following reasons: first, swollen lymph nodes in the armpit, such as bacterial infection or viral infection, can lead to swollen lymph nodes, some other causes can also lead to swollen lymph nodes, such as autoimmune system diseases, blood system diseases, or even malignant tumor lymph node metastasis, etc. This requires further investigation of blood routine, ultrasound, etc. for clarification, and if necessary, lymph node biopsy is required to confirm the nature. If necessary, lymph node biopsy should be performed to clarify the nature. Second, sebaceous cysts, which can be secondary to infection, are caused by blockage of hair follicles and need to be surgically removed to be cured. Third, the formation of parametrium, which is common in women during pregnancy and postpartum, generally do not worry too much, as the body hormone level decreases, the swelling can gradually subside, but the breast swelling continues to increase or pain will need to be surgically removed.