So, its looking more like the watch. Just for kicks, flip it over and let it read on the underside of your wrist. See if its any different.
I did that 2 days ago when I was going for a walk. The watch was stuck in the 60's but my chest strap was showing 130's. I flipped the watch around and it jumped up to 140. I didn't keep it there long enough to get more readings.