Whenever I listen to my heartbeat, I notice it goes through fits of erratic beating or skipping. What's up?

We have a few stethoscopes lying around the house (don't ask), and when I lie in bed I can hear my heartbeat through the pillow. I notice that my heart beats normally, but after every minute or so (sometimes less than that, sometimes up to five, possibly eight minutes) something strange will happen to the beat. Sometimes it'll sound like it skips a beat or two, it sort of stumbles I guess. Other times it gets a lot slower for a few seconds, then returns to its normal rhythm. Other times, and this is the most common, it'll suddenly speed up, like REALLY speed up and go super fast for a few seconds and then returns to normal. I'm not sure if it's hereditary or not, but I think my grandfather may have had irregular heartbeat (I'm not positive!)... though I thought that condition meant the heartbeat was irregular constantly, not sporadically like mine. I'm only seventeen years old. I've noticed this for some years now and have just thought to ask about it.