It could also be a live wallpaper. Recently I downloaded the Super Mario Live Wallpaper. While it is pretty cool, I found that my phone gets really hot when it would just be sitting on the desktop. Hot like I was playing Need For Speed Most Wanted.... but I wasn't doing anything. After just a few minutes of sitting there watching it, I noticed I lost about 4% of battery.
Certain apps and live wallpapers can be programmed very bad, and cause a ton of strain and drain on the phone.
If you can't figure exactly what it is, do a factory reset. If that does work, it was an app. If it doesn't, then as another said, you could have somehow got a faulty battery. If it is the battery, you might want to just invest in this one, as it is a lot better http://www.amazon.co...e/dp/B008GWIVTQ