Apple Watch heart monitor

I have used the 3 series watch for 2/3 years and the heart monitor has always read between average of 155bpm and 175 bpm when I have been out on a run. I got a series 6 about 3 weeks ago and the heat monitor is all over the place I have had 3 runs with and average of over 200 bpm and this morning the average was 93 bpm (on a 6k run with an average pace of 5.32 per km). I am be a regular runner and the average heart beat has never varied this much is the watch faulty?