Quickly double tap the button on the bottom.
Check which Apps you have running in the background.
Hold your finger on one until they all start wiggling. Hit the "-" sign to turn them off.

That's probably what's draining your battery.

Alternative method it to totally power off (hold top button down until you get the red scroll and shut down).... and then turn the iPhone back on again. That shuts all apps down.

Had the same thing happen to me until someone showed me. blush
David (OFI)