Pain is some sort of stress on body tissue, though it could be anything from intercostal muscle strain (muscles around ribs), to heart pain (angina), to pain originating from somewhere else (kidneys, for instance) that you just feel in your chest (refered pain). "Tightness" is a bit different. Usually a tightness in the chest is caused either by A) Constricted airway, reducing alveolar gas exchange; or B) Respiratory alkalosis, which can cause a tightness and sense of urgency (like you can't breathe). I would advise you to make sure you aren't hyperventilating when this occurs. Do this by deep breathing only once every 6 seconds (in for 3 seconds, out for 3 seconds) for at least 3 minutes. In this way you will maintain (or return to) normal pH, while still supplying yourself with enough O2. If you still get the feeling, it is probably not alkalosis (the least serious), and should be further investigated if it is causing discomfort.