First of all, please ignore the nutcase suggesting a colon cleanse. =)
This advice is coming from a registered nurse with a good amount of cardiac experience.
Please see a doctor. It sounds like you are having palpitations, and they can indicate a possible problem. Assuming these episodes are not occurring with exercise or emotional stress, they can mean a number of different things. I won't get into complicated details, but it could mean your heart has episodes of beating too fast or in an unusual rythm.
See your regular primary care doc, and he'll probably have you set up on a cardiac monitor for a couple of days to see if he/she can catch an episode.
Good luck and best wishes!!