Under my left breast bone its tender like a bruised feeling. no marks on skin. when i cough/sneeze it hurts a lot in that area. over a week now.

Chest muscle strain: from coughing. If pain is severe you will need a chest x-ray and exam; otherwise, use ibuprofen ( Advil) or naproxyn sodium ( Aleve) assuming that you are not allergic to NSAIDS or any of these medications.

Cough: Cough has several causes, and if it persists or interferes with breathing please see your physician. Keep nasal passages clear with saline or a decongestant.