The causes of half breast pain include cystic breast hyperplasia, acute mastitis, and acute lymphadenitis of the armpit, which are as follows: 1) Cystic breast hyperplasia: it is a common benign breast disease among women of childbearing age, manifested by breast pain and accompanied by the formation of bead-like nodules, the pain is mostly tingling or distending and the degree of pain is related to the menstrual cycle; 2) Acute mastitis: it is commonly found in breastfeeding women who suffer from an acute suppurative infectious lesion in the breast. Infectious lesions, manifested by the affected side of the breast swelling and may be accompanied by severe pain, and even in the breast can be touched fluctuating mass; 3, acute lymphadenitis in the armpit: manifested in the armpit red, swollen, hot, pain, when the pain is further aggravated can be radiated to the ipsilateral breast caused by the same side of the breast swelling and pain. There are many kinds of diseases that cause pain in half of the breast, you should go to the regular hospital to have a clear diagnosis and timely treatment to avoid further aggravation of the disease.