What to do about sharp pain in left breast when taking a deep breath?

Pleuritic; costal: Costochondritis may be caused by trauma, such as bumping the rib cage; as a result of a viral infection; or as part of an inflammatory disease process. Usually the cause is unknown; pleurisy is inflammation of the lining of the lungs and chest (the pleura) that leads to chest pain (usually sharp) when you take a breath or cough.

Deep breath/pain: A sharp pain which actually stops the respiratory effort because of the pain, but when the breath is slowly inspired, disappears is probably "pleuritic" (lung) pain, caused when shallow breathing (while sitting at a desk, for example) allows the unexpanded lung surface to stick to itself like a collapsed balloon. A deep breath at that point causes pain as the lung expands.