Hot hot hot
So about maybe once a week i notice that both my iPhone 5 (iOS 9.2.1) and/or iPad Air 1 (iOS 9.2.1) get really hot. It always happens around sunset, when f.lux starts "waking up" the screen...
(both non-jailbroken devices)
If it's once a week or less I'm guessing it's another app or tweak you're using.