I had a sharp pain under my left breast bone/rib area that is sore now when i breathe in. i read that could be a myriad of very dangerous things but when i went to the er they didnt find anything. what could it be?

If you do not have any other symptoms like cough, fever or shortness of breath, you probably have costochondritis, a benign inflammation at the connection between the rib and breast bone or cartilage rib. You can take Advil or Motrin or Ibuprofen OTC 200 mg x 3 tablets with full stomach, every 6 hours, for 2-3 doses. If the pain goes away, you probably have this condition.

Majority of such conditions prove to be musculoskeletal in origin, otherwise hard to prove with imaging studies. In ER efforts are made to eliminate possible life threatening conditions. That's why follow up is usually needed, ideally with your doc, in order to have a good plan going forward, that may include different tests or a therapeutic trial.