Pain in the left breast against the armpit is associated with mastopexy, paronychia, and acute lymphadenitis. If you have pain in the left breast, you can first check yourself. Local hard lumps are considered to be caused by mammary hyperplasia. Increased hormone levels before menstruation and hyperplasia of the lobules and glands of the breast can cause pain in the area of the lump. It can also be considered to be caused by axillary parametrium, which is affected by menstruation, and the local lymphatic system is not able to detoxify due to hormonal stimulation before menstruation, and the internal condition of the parametrium will form a lump, which causes various local pain due to body and parametrial stimulation. As a result of trauma and local infection in the armpit, Staphylococcus aureus invades and causes local lymph node enlargement and inflammation, which causes swelling and pain when pressing on the armpit.