Probably an arrythmia. Having some once in awhile is normal really if they dont happen often and dont last long. After my heart attack I will have some that last 5 minutes for more and I actually passed out about 1 week ago sitting at the computer playing GTA4. I was driving a stolen cab and running from the cops at high speed. When I woke up from the arrythmia I was lying on the floor and I had crashed my car into a parked car! Thank goodness it was just a simulation game and not real life.
Anyways, next time it happens you should feel your pulse. Is it nice and strong or is it weak and hard to find? It could be a tachycardia where its beating really fast or it could just be a PVC. PVCs are where the pulse is irratic and you skip a beat. When you skip a beat there is more time in between the pulse you feel and then the next beat is stronger than normal. THe stronger one is called a compensatory beat. Having PVCs once in awhille is normal.
Have you checked your BP lately?