Apple Watch Workout app keeps ending workout immediately after starting the workout.

I've havd my Apple Watch, Nike version for about a year. Today when I used the Workout app to start a walk, it did the 3-2-1 countdown then almost immediately vibrated and asked "Finished your workout?" I dismissed the message and it came back up again right way. Can't get it to stop. I rebooted the watch twice, but it has not resolved the issue. Suggestions in the community point to the watch being too loose at the wrist, which it wasn't, but I tightened it just to be sure. I have been wearing the watch and Workout app the same way for a year for walks, runs, and bike so can't figure why it would not work now. Any suggestions?