It's normal for everyone's heart to 'skip' a beat.. and sometimes feel like it's fluttering... several things can make it worse such as caffeine and stress
There is a disorder called MVP
(Mitral Valve Prolapse
). I have this and it's usually harmless.
You should tell your parents what is going on and let your doctor listen to your heart just to be on the safe said... unless he checked that specifically and didn't find anything. The chances of it being anything serious are slim, but you should always get it checked.