The common causes of hard lumps in the armpit after childbirth: 1. Acute mastitis, due to breast-feeding distension or improper care after delivery, some women may experience breast swelling, local redness, pain, and in serious cases, swollen axillary lymph nodes, painful to touch, this condition is mostly caused by infection, patients are often accompanied by fever symptoms, routine blood and breast ultrasound can be clearly diagnosed, anti-infection treatment is the main focus, if necessary, the abscess needs to be performed. 2. Breast cancer, because of the early symptoms are not obvious, after pregnancy, the body hormone level is abnormal, the tumor grows faster, some patients can find a lump without pain after delivery, and the demarcation with surrounding tissues is unclear, ultrasound of breast and axillary lymph nodes can help to diagnose, to confirm the diagnosis, puncture biopsy is needed, the treatment is mainly surgery, radiotherapy and symptomatic treatment are given after surgery. Treatment is mainly surgical, postoperative radiotherapy and symptomatic treatment. The younger the age of onset of breast cancer, the more malignant it is and the prognosis is relatively poor, so when you find a lump in the armpit, you must pay attention to it and seek medical consultation in time.