Vivoactive 5 heart rate too low

Been using my vivoactive 5 for a month on indoor cycle.  Heart is in my normal expected range 130bpm. However for the last 3 rides it is excessively low 90bpm,  even though i ride harder to elevate the rate,Tried changing Arms,  Cleaning sensor and even using the chest strap, but no luck. At the end of the ride it increases to what I would expect during recovery, 110. Any ideas