Or, it could be....
Premature Atrial Contractions (PAC's) or Premature Ventricle Contractions (PVC's). In that case, those chambers make two beats really close together. Since the second beat is too rapid, the chamber doesn't fill up with much blood, and it's a longer time til the next beat. So, that means that beat has almost double the normal amount of blood, and that extra gush is what's felt. Kind of feels like a fish tail flopping in your chest. PAC's are not considered to be very bad, PVC's are much more worrisome. I have PAC's, the doctor told me to cut my caffeine down to a level that they wouldn't bother me. I cut my coffee down by more than half, about 10 years ago, and they mostly went away. I still feel one every once in blue moon. Dehydration seems to trigger it as well. My Dad has PVC's and he had to give up caffeine completely.
Hope it all works out okay.