Why is my iPhone 5s getting warm/hot?

Recently, my iPhone 5s started getting warm and occasionally hot when I use it. I'm trying to find the source of the problem. It began by getting warm/hot when I would charge it, and then my charger stopped working, and I had to buy a new one. I also got a storage almost full alert, but my friend's storage is full, and her phone doesn't get warm/hot. It immediately cools down within a minute of turning it off. I have only 5 apps that aren't the ones that initially come with the phone; the only one I use quite often is Tumblr. Do you have any idea what might be causing it?

Added (1). The phone heats up after very little use and hasn't been left in any hot areas.
I haven't used the new charger yet, but it is not an Apple one (but the one that broke was an Apple one).

That happens to me a lot too and i have an iphone 6. Usually it happens because it is out in a warm area or sunlight for too long. Or it means you've been using it for a long period of time.

Did you buy a charger from the carrier or App! E? Many times those cheap chargers cause all kinds of problems, not to mention can void the warranty.

I would factory reset the device after backing it up and the issue will probably go away.

What do you expect it's an apple device

Turn off the options you don't use much. Like Bluetooth, Wi-Fi, Locations etc. This saves battery and help extend its life because the charge recharge cycle lessens automatically.