That can happen sometimes if your watch is too loose or too tight or your phone or watch restarted or updated overnight. If it's a one-off thing, I wouldn't worry about it, but if it continues to happen, you should put in an error report.
Could have been a syncing error. Had you looked for the score on your watch? I'd only be concerned if the stats didn't appear on the watch, as that would mean the sensors had not been working properly or were hampered in some way.
