No, chances are there are some background apps running. Install CPU tracker such as watchdog lite, etc. Set CPU warning for any program exceeding....let say 40%.

When your phone suddenly go very hot, check the CPU program and do real time tracking immediately to see what program is currently running your CPU down. Last time stock video media did that although I don't use them. If its your installed apps doing that and its not a critical app, uninstall them.

If you fully charge your phone and it can't reach 100% and remained hot after a while even after charging complete (charger still connected, trickle charging), some apps are definitely running your battery down on the background.

even at 100%, it's still charging, until the green icon comes out at notification bar saying that battery fully charged, then led goes green.

technically shouldn't overheat when charging, I mean, it'll be hot la, but overheat?

When you said overheat, what was the temperature ? Above 45°c ?

The phone will go into safe mode or auto showndown before it become overheat. Don't worry bout the temperature.

I think you mean only hot, not overheat. Normal when surfing or using apps like games. Based on what you say on off charging, definitely some background apps is running. When your phone is fully charge even overnight, your phone will be cold during trickle charging after that.

If it is still hot, one hour after charging complete, background apps is seriously whacking your battery. Check either watchdog lite for current CPU usage (if very very hot) or battery stats tracker like Gsam battery app to see what program uses the most battery even when you're not actively using them. It will gives you clue to what apps to uninstall and check to see if its helps after that. If it still happens, continue do the same when uninstall next apps that consume battery when you did not activate them.