Hello~I hope that by now you have seen your doctor and that all is well. Sometimes, gas, can indeed, cause these pains, and the reason it is sometimes after exercise is that you have moved the air bubbles around just enough to push on some of the heart or areas around it.
Your blood work looks fine.
Are you overweight, out of shape? The best option would be a cardiologist who can possibly conduct more tests, perhaps a Holter monitor if needed, and find out if there is anything showing during the actual episodes.
Well, the good news is that you have been seen by a doctor. However, always an ER doctor. The problem with that is that they treat an acute situation and release you. What about a physical with a general practitioner that can help you with an ongoing problem? Perhaps it IS just gas. Not sure why exertion would cause that. But you would be best to get a full physical. It's likely not a heart issue as it is cleared by the time you get to the ER (and the heart doesn't really work like that). Let us know if you can get a physical (and that should be much cheaper than visiting the hospital ER each time)!