The upper left part of the chest, near the armpit, is the location of the lateral border of the breast, the parametrium or the pectoralis major muscle. The more common cause of breast pain is cystic hyperplasia of the breast, which is a swelling and stabbing pain due to excessive hyperplasia of the breast tissue caused by hormonal metabolism disorders in the female body. Acute mastitis can also cause localized pain, and because the parametrium has normal breast tissue, it can also develop cystic hyperplasia of the breast, which is also a common cause of parametrial pain. Pain in this area can also occur due to muscle soreness caused by excessive strain on the upper limbs or pectoralis major muscles, which can generally be relieved with local heat and more rest. If the local impact, extrusion and other traumatic conditions can also cause pain, try to go to a regular hospital, check to confirm the specific situation, timely symptomatic treatment.