Yesterday when I pulled mine out to read while I was eating dinner I found the dead battery screen despite the fact that it should have had about 95% at the time. There's only one thing I can think of: I had just spent several hours in areas with no cell service--it must have been keeping trying to find a tower to see if I had bought anything.
I keep the wifi off at all times unless I am going to download a book.
That being said however, I had a Kindle with a bad battery. Since I don't keep the wifi on, the battery should last but I was reading and watched the battery lose its charge until it was almost empty in a matter of just a couple hours. If the Kindle is still under a year old, Amazon will send you a replacement straight away.
If you just downloaded a bunch of books, it was probably the indexing that caused the battery to drain, though even that in a few hours is not normal (or you are stuck on one book)
And more importantly with this, Kindles have been known to get stuck while trying to index books, which can drain a battery bad. There's some process involving doing a search that lets you see what hasn't been indexed, and by watching that, you can get an idea of whether it might be stuck or not. If it is, generally removing the book and readding it can solve that problem.
I've read several accounts of people having problems with batteries early in the life of Kindles. One person I read of even had to get to the third swop-out, before they got a Kindle that did not fail. May have been earlier Kindles, but read the reports fairly recently.
I wouldn't delay in asking Amazon to swop it out. Better now early in your ownership. And check next one carefully in case you are one of the unlucky few!