What is causing my ipad and iphone to get too hot?

My iphone was just in my bag, fully charged. When i went to retrieve it, it was searing hot i thought it was going to explode. At the time it was only 6 months old and a 4s model. After half an hour it shut down by itself and cooled off. Now i have a new ipad 4th gen (A1460) and after fully charging, it went all haywire, piping hot, would not turn off, then completely flattened itself within 20 minutes. I don't know what's going on. I let both devices run flat to drain battery at least a couple times a week. Any advice greatly appreciated!

Same, own both.
Same generation.
However, I got no problems with overheating.
Did you know: if you leave apps open when iOS turned off they still run?
So that fun fact is true, close all apps before turning off.
And don't keep it in small areas.

Keep your devices updated. Turn off apps. Plug directly in a socket and avoid plugging Iphones into computers if you can manage that. Remove case when charging. If it gets too hot, stick it in an air-cond for a few seconds, it will cool it down in a matter of seconds.

The heating can cause bugs, slowness or fast battery drainage (mostly because heat is energy being wasted or used inefficiently… ). If you still have problems, take it to a store.
Oh, and when you charge, make sure your iphone is under 20% battery and charge overnight or something. Apparently, that helps with the battery life.