Information from an old thread that I think explains your question well:
Hi! They sound like premature heart beats. They are commonly called pac's or pvc's. PAC means it is the upper chamber contracting too soon. PVC means it is the lower chamber contracting too soon. They are NOT stopped or skipped beats. They are EXTRA beats in between the normal rhythm beats. You dont feel the extra beat but normally the next regular beat. That is when the heart has a bit more blood in it that it is pumping out so you might feel the thud or little pound. Actually it can be a little butterfly flicker or a big bunny thump! ( That's what I call mine.)
You cannot tell the difference between a PAC and a PVC although some people say they can. You can only tell the difference by being on an EKG. Just about everybody gets them but not everyone feels or notices them. (Oh why do I have to be one of the lucky ones who notices??? )
The reason they might 'feel' different from time to time depends on when the EXTRA beat occurs in relation to the next regular beat. If it is close the the next regular beat you would probably only feel a little flicker or thump. If it is close to the first regular beat and farther from the Next Regular beat, your heart will fill up with more blood and you will feel a real THUD when it contract to release the blood into your system. STILL BENIGN in a structurally normal heart.