There is a bag under the armpit is caused by the following reasons: 1, body surface swelling, such as lipoma, sebaceous cysts, fibroids, etc., need to go to the hospital to improve the relevant examination, such as blood routine, ultrasound, etc., if it is a lipoma does not need to be excised, if it is a sebaceous cyst, you need to improve the relevant examination as soon as possible for excision to prevent ulcers and infections; 2, may be the formation of paratesticular breasts, commonly found in pregnancy and postpartum With the end of breastfeeding period, the lumps can gradually subside, if they don’t subside or even continue to increase, they need to be surgically removed; 3. It may be caused by enlarged lymph nodes, and there are many triggers, such as bacterial infection, viral infection, or even lymph node metastasis of malignant tumors, etc., which need to be further perfected with relevant examinations, including biopsy to clarify its nature.