it doesn't happen when charging, it happens when I use some apps.  It happens with Pokemon app, with Googlemaps app, with some youtube app, sometimes online using either browser etc...  It has happened a lot actually.  It happens a lot when I am outside, even though I keep my phone in my purse, I have had to use it for the maps when walking before and it has overheated.  It happens when I'm roaming too.  It isn't constant, it just happens now and again.  I usually turn off my phone if I can when it gets really hot, or I cool it off in from of an A/C vent.