Cardiologist says I have premature irregular beats but I know the monitor didn't catch the really fast twitching heart beats. What are those?

PAC, PVC. A momentary feeling of a skipped beat is usually actually an early beat from the upper chambers (premature atrial), or lower chamber (premature ventricular). If it last longer than a single heart beat, it may represent a tachycardia. A longer duration heart monitor, such as an event recorder may be needed to actually catch it. Ask your cardiologist if this is necessary.