After excluding the local muscle strain caused by trauma, the feeling of soreness in the armpit may be caused by acute mastitis, breast hyperplasia, swollen lymph nodes in the armpit and lipoma. 1. Acute mastitis: Patients with acute mastitis will experience breast pain, localized redness and swelling, accompanied by swollen and aching lymph nodes in the armpit on the affected side. 2. Mammary hyperplasia: Mammary hyperplasia is usually associated with the menstrual cycle. Diffuse nodules will appear on the breasts, and the breasts will be obviously swollen and painful during menstruation, which may also cause soreness and swelling in the armpits. 3. Swollen lymph nodes in the armpit: When inflammation occurs locally or in other parts of the body, it can cause the lymph nodes in the armpit to swell, resulting in swelling and discomfort in the armpit. 4. Lipoma: when lipoma occurs in armpit, if it is small, it usually does not have obvious symptom and feeling, but if the lipoma is bigger, it may appear local compression, thus the symptom of underarm swelling and discomfort occurs. When the patients have the feeling of soreness and swelling in the armpit, they should go to the hospital immediately for consultation and reasonable treatment under the guidance of the doctor to clarify the cause of the disease.