My heart feels like it stops then beats real hard only once after that. It's happening several times a day.?

PVC. What you are having is a premature ventricular contraction (PVC) where the heart has an early beat then a pause as it falls right before a normal beat so the normal beat doesn't occur. Then the next beat takes a little longer to occur. During that time the heart fills just a little more so the next heartbeat is stronger and thats what you are feeling. It is called starlings law, heart beats harder.