I find that this sometimes happens after updates, but it often clears up on it's own after a day or two. With this update, I had some battery drain on Tuesday and Wednesday, but Thursday was fine.
When it gets very low, it will ask to go into "power reserve". You should turn that on, then charge it. Sometimes that's all it needs, and after it charges up again it will be back to normal.
If that doesn't work, you can try unpairing/repairing your watch and phone, but that's a last resort. It takes a while and is pain in the neck, so just waiting to see if the battery issue goes away by itself (or if there's another software update to fix it in a few days) is a better option at first.