Left breast pain against the armpit may be caused by breast hyperplasia, acute lymphadenitis, soft tissue injury and other reasons, the specific causes need to go to the hospital for examination to determine. 1. Mammary hyperplasia: Mammary hyperplasia is generally related to age, endocrine dysfunction, estrogen and progesterone imbalance and other factors, the patient’s early unilateral or bilateral breast will be slightly swollen and painful, with the development of the disease patient nipple will appear overflow, some patients will also have menstrual disorders. 2. Acute lymphadenitis: acute lymphadenitis is usually caused by infection, cold and other reasons, mostly in the neck, armpit, groin, inner elbow and other parts of the body, the condition of patients with mild local lymph nodes will be enlarged, pain, severe condition will also appear fever, severe pain, skin redness, headache and other symptoms. 3. Soft tissue injury: the left breast by the armpit has more muscles, if the force is not appropriate to make the local soft tissue injury, the patient will have pain, swelling symptoms, advanced patients will also appear muscle tendon adhesion, ischemic spasm and other symptoms. There are many reasons for the pain in the left breast by the armpit, if the pain is mild, you can first observe for a period of time, if the pain is severe, you should go to the hospital in time to consult a doctor, after diagnosis and treatment as prescribed by the doctor.