It's a very common symptom for people to express concern about.  Usually it's nothing to be concerned about.  Sometimes the fluttering is the heart, other times it's not, despite how certain you are that it is your heart.  Regardless, try to cut back on caffeine, alcohol, and stress to see if this limits the frequency and/or severity of the fluttering sensations.

You should seek medical assistance if the fluttering lasts a long time (unclear definition, but perhaps 15-30 minutes would be a "long time").  Also, seek medical assistance if you get other symptoms (e.g. lightheaded, dizzy, short of breath, chest pain, etc), even if the fluttering is only a couple minutes in duration.  "Medical assistance" in this situation is usually defined as ER or urgent care evaluation, not waiting until a day or two later.