I have pain on the left hand side, just under my left breast bone, when I breathe?

Pleurisy. Well localized pain in your chest (rib cage) after taking a deep breath is often due to "pleurisy" a viral infection (cold) that gets into the lining of the lung, producing pain when the inflammed surface of the lining of the lung rubs against the ribs when you take a deep breath. The same thing can happen after a contusion or bruise to your rib cage.