From what limited information I know (so you should probably want to check this out with your doctor and most of where I get this from comes from preventing high blood pressure problems - that aren't even on me - so it could again be completely different) there are a number of different medications that act in different ways. Basically you have a chain of events, or a pathway of chemical reaction occuring that makes your heard beat faster and harder. Different events can act on different parts of this pathway and so can have different side-effects.

Personally, I would stay away from the very first doctor you saw - every doctor should always tell you the risks and side - effects of everything. That doctor is just an eejiit.

Basically it can happen to anyone, no matter how fit. Have a read about a guy named Greg Welch, retired triathlete and formally one of the fittest people on the planet. Probably didn't have the same thing as you but similarish.