That heartbeat you are feeling...could be a result of a heightened release of
epinephrine(
adrenaline) in your system, which, yes, is caused by anxiety...in my case, it was actually the cause of my first attack...
What's strange though is, usually, as far as I know (which I am not a doctor, mind you) exertion, i.e. working out/exercising, takes that adrenaline, which is coursing through your body, and is eaten up by your working muscles.