Sharp pains and pain under the left breast could be caused by many things such as gas; ulcers; stomach problems; gallbladder or pancreas. It is important to see your doctor so they can give you the proper diagnosis. You may just be given something for Acid Reflux (acid that comes up into the throat from the stomach) or something for your stomach.
One of your ribs might "be out". This means it is just out of place. You might just be dyhydrated, so drink plenty of water. If it doesn't go away with in the next few hours, you can either go on...
An internist may explain or identify the cause of pain. As far as I knew, alot of condition may trigger such pain. An acid influx due to acidity may cause it. It could also be just an ordinary muscle...