Had an early beat which felt like a thud in chest followed by a pause then normal beat. no thump followed, what was this?

Palpitation: A premature ventricular contraction (PVC) or premature atrial contraction (PAC). Sometimes a healthy heart has an early beat - that beat resets the timer and a normal beat follows. That sensation is often described as a flip flop. Sometimes a few in a row can give you a real start, but they are benign unless accompanied by other serious signs or symptoms.