I have had a pain under my left breast since i woke up this morning and hurts a bit when i do a deep breath. any ideas why this could be?

Chest wall: Watch it, and if it begins to take your breath seriously away, or starts to radiate or becomes severe get seen. Chances are it's a small fold in the pleura or other mechanical problem in the chest wall. Check and see if there is a tender spot over one of your ribs or the cartilages that attach them to your breast bone -- if there is, read up on "costochondritis" & if it gets worse, get seen.

Pleurisy: Could be an inflammation of the linning to your lungs known as the pleura. Take 2 aleve (naproxen) ever 12 hours for 2 days. This should fix you!