Rapid heartbeat is obviously a sign of sexual arousal, so it's probably fair to suggest that it's probably just that you had exerted yourself more than usual, or that you were more aware of it than usual having checked your pulse earlier that day.

I have to say, I'm fairly body conscious, check my breasts regularly, go for smears when I should etc but I've NEVER felt the need to take my pulse, even when I've been aware that my heart rate has increased. Not even since being pregnant have I felt paranoid enough to do that, so perhaps your anxiety about your physical health is actually what's causing you the problem. You've been checked out, you appear to be normal and your heart rate returns to normal quickly when resting. I personally don't think you have anything to worry about.

__________________
"Time flies like an arrow -
Fruit flies like a banana"

M Y - N A U G H T Y - P I C T U R E S ! !