theyre probably PVCs or PACs (premature ventricular/atrial contractions). basically, a cell in the heart fires to early which makes it beat twice really fast, then your heart pauses for a brief second to allow itself to get back on track. i get hundreds to thousands of them a day, it kinda sucks. 10+ a minute when they are really acting up. lots of people get them and theyre completely harmless in most people, but you may want to consider getting checked out sometime because they can occasionally be symptom of other problems. but im not trying to scare you, most lof the time theyre harmless, as mine are. some of the common causes are too much caffeine, lack of sleep, smoking or drinking, and stress. but some people like me just get them for no reason. if youre getting a lot and they really bother you, the doctor can put you on a beta blocker which helps reduce them in some people. you can browse through old questions in the heart forum at medhelp.org lots of people have asked questions on them and you can learn a lot from reading the answers