Why is my IPhone Overheating while its turned off?

Ok so like I set my phone down a while ago after skyping for a bit, and it sat on my bed for several hours before I picked it back up.
and when i picked it up it was so ******* hot some of the glue holding its case together melted just a little bit.
so I took the phone out of its case, and the phone is almost too hot to touch right now.
I read that if its over heating you can reboot it and it should be fine, but the phone won't turn on…
I don't know what to do i hope its not broken…
How is it overheating while its turned off? This makes no sense…

its sitting near my air conditioner right now… But I'm scared that its going to get too hot and **** something up inside…
I mean its not SUPER hot right now, but you can't hold it for very long before it starts to hurt…
If i could actually measure its temperature then i could give you a better idea of how hot it is…

Added (1). By the way it is an Iphone 5s Gold, i have had it for about 5-6 months now.

It sounds to me like there might be a short somewhere that could be causing that. I think it might be the battery. I had a small walky talky that got like that at one point. It wasn't running but it got really really hot. I could hold the walky talky but I couldn't touch the battery.