"My cardiologist says the fact that it goes away is a mystery. Another cardiologist suggested that maybe endorphins mask the pain."
Keep #`1, fire #2. Surgeons used to not give people with appendicitis morphine for fear of "masking the pain" and thus missing the diagnosis. Utter crap. The fact that it goes away means your body has found a way to compensate for the flow. Cardiologist #1 realizes that but doesn't understand why. Good, that's ok, we don't know everything. Cardiologist #2 is making up a crap answer instead of saying "I don't know" - makes no sense. Reality, doesn't always make sense.
When I run, I routinely get an endorphin kick around 17 minutes in.