Ok that kind of premature contraction is a mystery to science. If you are healthy then you can ignore it unless it does not stop . if you are not healthy and have a preexisting heart condition that type of premature contraction can lead to ventricular fibrillation and instant death because no blood to the lungs you have about 7 minutes before trouble starts. So if you can ,go the doc for a heart scan.

What you are feeling if it is premature ventricular contraction is the bottom of the heart which pumps blood to the lungs pushes too early so the heart system says Stop the process and do the whole cycle again. Kind of like a dance coach not liking the dancing. Stops and says do it over again.

Watch for multiple thumps in a row and shortness of breath