I agree with the above. As long as your heart is structurally sound, a few flutters aren't going to kill you, especially if it is only a few. Your heart is probably just sensitive. That being said, make sure you're not exercising too hard and stressing your heart unnecessarily. A general rule of thumb to follow is that you should be able to carry on a light conversation while you're exercising. If you're so out of breath that you can't talk, you're probably going too hard. Everything in moderation: believe it or not, that includes exercise!
I'm not sure I would call it 'normal' but it certainly happens to a lot of people and is usually benign. I suppose it depends on each individuals subjective view of what is normal but I would say the average person probably does not have heart flutters when exercising and cooling down.
In any event, since you've had the appropriate tests and consulted a doctor, I'm sure he/she would have told you if you had exercise restrictions so sounds like you're in the clear to exercise away and put the flutters out of mind. Anxiety can definitely make heart palps worse so if doc says you're fine, I would trust him and try not to worry.