That has happened to me and all I did was shut the watch completely off and then back on and all was well. You are definitely not the first but most have been able to fix by shutting off and back on. Also, check on the Apple Watch App to check the usage. Maybe you have an excessive amount of usage that you are certain you didn't do and that could be a bad app or something else that no one can explain.
Most of the time, a cold boot fixes the problem, removing some third party apps and adding back to see if one in particular is causing the problem. Hopefully you won't have to do more but there are plenty of posts even as recent as yesterday.
A month or so ago, I posted that my watch was about 90% and after mowing the lawn it was at 20% and I didn't know why. I charged back up and it was going down fast again so I cold booted and it was back to normal.
I now watch daily for suspicious drain and end up cold booting every few days just "because".