You are not alone. I am seeing tens of thousands of users (including myself) reporting battery stability issues on various Apple watches after updating to Watch OS 10.1. There is clearly some sort of bug or complication within the new update that excessively drains the battery.
Some common recommendations I’ve noticed are to uninstall all third-party (non-Apple) apps from your watch, and to charge on Low Power Mode. These recommendations clearly hinder the convenience and efficiency of owning an Apple Watch, but it will at least prevent potential damage to the battery’s overall health.
This is absolutely insane. This update has been available for some days now and Apple has yet to release formal communication that apologizes for, or even acknowledges the battery problems with 10.1.