Does the dr mention anything about it?
Adrenalin, spasms, hyper thyroid??

Can you sense it or feel it when it's doing it?
Any other things that you notice when it's happening?

I had a couple of episodes where my heart rate seemed fast - I could feel it beating in my chest like a vibration. I had quick dizzy spells along with it too. What I had wasn't constant it would come on and last about an hour or so, for about 2 week time span then it would just fade away.