There are many different causes of pain on the lower side of the breast, and there are several specific causes: 1. Injury. If you are pressed, hit or traumatized, it can make the lower side of the breast painful. If there is no wound, no fracture of the ribs, you can put plaster for massage, if there is a wound you can put a band-aid, a large wound needs to be sewn on. 2, rib fracture. For example, car accidents lead to rib fractures, some only fracture cracks, some ribs completely separated, need timely hospitalization for fracture repositioning and fixation to prevent deformation healing. 3, skin allergies. This can be accompanied by skin pain, itching or redness, and requires anti-allergy medication.4. Breast enlargement. If there are regular painful nodules on the lower side of the breast, it may be mastocytosis, which will disappear after the end of menstruation, and severe pain requires medication for pain relief and endocrine regulation.