I have the same thing. My dr. did a blood test and I am anemic. My iron level is low and he said just going up and down my stairs would cause my heart to beat fast too. I don't get it while exercising because you build up your heart rate slowly. When you just run up or down stairs its a sudden fast heart rate. If you haven't already have a blood test done to be sure its not something like that. I have a lot of stress and it makes it worse.
also...how are you sleeping at night? Do you drink a lot of caffine? Have a lot of sugar intake? I have heard that these can be factors in increased adrenaline after your body has experienced a sever panic attack?
Are you anxious, stressed out? Or are you simply experiencing a more noticeable heartbeat?
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.
I am not sure if this is something a psychiatrist might be aware of, since a lot of psychiatrists are educated on the physical effects of anxiety, but hang in there, give it some time, and check to see if there might be some short-term meds that might help.