For about a week now, dull sharp pain behind left breast and behind sternum on and off. its scaring me ?

Needs a doctor exam: In a college-aged person, heart problems are less likely, but rib or sternum joints may hurt for many reasons, such as trauma (bumped into something, somebody poked a finger into the ribs), coughing (bad cold and cough symptoms), jogging or running (breathing heavily for a while), weight training or sit-ups (over-exertion while exercising), irritation in the rib's bones, muscles, cartilage, etc...