There are quite a lot of possibilities but a collapsed lung is probably more likely than anything heart related and neither is likely. Ok, first thing about pain is finding where the problem is. If poking a muscle makes it hurt, that is the problem. Probalby the most common spots are the attachments of the ribs to the center of the chest. if poking the rib joints is the source of the pain, then on the one hand, it isn't serious, on the other it takes quite a while (weeks) for the pain to dull down.
Pain on exhalation fits with asthma, often mild and without wheeze. But a simple doctor visit with listening to the lungs would probably tell what is going on and lead to someting to fix it.