It could be a late ectopic beat. In the setting of PVCs, they may "merge" with the next normal heart beat, so-called fusion beats. A normal and benign phenomenon.
It could be a so-called escape beat; another "pacemaker" in the heart fires a heart beat because your heart rate is slow. This function is essential because it prevents your heart from stopping if your sinus node for some reason should fail. The heart has a lot of "backup" pacemakers (which is the reason why we get PACs and PVCs). Also usually a normal phenomenon at night, but ask your doctor.
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