Just seeing the thread title, and going no further, my thought was "heart" - get to a MD Man. You are describing the class FAIL in a stress test

That said, I'll say that I occasionaly also get/got (hasn't happened in years) a SHARP pain right in the sternum - yeah, went to the hospital, and they found a neuroskeletal problem, I moved JUST the right way, I'd feel a "pop", and all was well - seems in certain weird conditions, I could catch a muscle behind a ridge on the sternum, and BOY would it hurt until it got "unstuck"