This has been happening to me for 3 months, I got it in May. Since 11/24/16 it has happened 6 times and the battery has never been lower than 45%. Apple has NO clue. At first they said they had never heard of my problem. Then I sent it in and they couldn't recreate it so they sent it back. It kept happening so they sent me a new watch after hours of being online with Apple customer service, to no avail. It has kept happening even with the new watch, so then they blamed it on my phone (also an iPhone) I am so frustrated I am ready to throw it in the garbage.
How can a company as large and technically apt as Apple not know what the problem is or how to fix it?