Hang in there, my friend! I know you must be really frightened and upset right now, as your body is not acting the way you want or expect. Try to breathe deeply. Are you having any chest pains? Is the shortness of breath still there or the irregular heartbeat? Do you feel dizzy or lightheaded? Those things could be deserving of a trip to the ER just to be safe.
If everything is settling down, and you don't feel a trip to the ER is imminent, write down what you're feeling and when it started. This will be a big help next time you get to meet with your doctor. I've noticed that when I have a lot of health anxiety going on, I think I'll be able to remember all the symptoms and details, because they're sooooo frightening. But then there get to be so many episodes that I start worrying that I'll forget something really important. Writing it all down means I can let go of the worry to remember, and I can also hand the list to my doctor and let him read it slowly; I'm more sure he's not overlooking anything that way too. I kept a running list of my symptoms, almost hourly, and things I thought might be related (like what I'd eaten, for instance). It got to be embarrassingly long and detailed, but it did ultimately help with the HA.
Also, you don't take any medications that would prolong or enhance your activity, do you? What I've heard on TV indicates that rare side effects can include a man having trouble settling back down after being active. I know nothing about any of that except for TV ads, though, so please take it with a big grain of salt.
By all means, bring up your concerns with your doctor next time you see him/her.