In my opinion
Collapsing, dizzyness, shortness of breathe, heart problems while exercising = heart problem
But then again, those could also be due to anxiety. The only thing is most people who only have an anxiety issue don't faint or collapse.
See with me my heart flutters/ palpitates when i'm nervous and thinking about it, but NEVER during any physical activity. That should be a warning sign right there. it's NOT GOOD if this happens during physical activity, as your heart is already working overtime to get oxygen to your body. One flutter or palp can send it out of whack and it wont be able to catch back up.
Since you have seen a cardiologist and your heart is fine, it could be that when they were looking at it, these "flutters" didn't happen then, thus they didn't catch it. Did you have a stress test done and also wear a holter monitor so maybe they could catch one in action?