Unexpected pain can occur under the left breast for reasons that range . sudden, severe pain under the left breast and in the upper center part.
Under and around the left breastbone are the heart, spleen, stomach, sharp, stabbing pain, often on the left side of the chest; sudden onset.
While your heart is under your left rib cage, feeling pain in that area usually of the cartilage that attaches your ribs to your breastbone. It causes a sharp, stabbing pain that's usually felt on the left side of your rib cage.