Well, if your doctor isn't concerned, it may be something of no concern. Getting a diagnosis off bikeforums isn't a wise thing to do- that's like saying "ow, I think I broke my arm! What should I do, guys?". I wouldn't ask anyone here to give me a medical diagnosis or ask their opinions. I'd see a doctor.
If you're really concerned, you'll want to see a cardiologist. They'll be the ones who can run specialized tests and look to see what the cause of this high heart rate.
Good luck, man.
Koffee