Phones getting hot when they are in constant use is nothing new. On my old "dumb phones" if I was on the phone for a bit I would feel the phone getting warm/hot. Every smart phone I have had has always gotten warm/hot when using it non stop (surfing the web, playing game, etc), even my wife's iPhone does this. When you are using it non stop you have a lot of factors going on in such a tiny package with no ideal cooling. Between the heat from the screen, processor, memory and the battery as it is being discharged, you get one hot device, this is normal.
If the phone is getting hot just sitting in your pocket then you have an issue with an app that is keeping your data radio open where it is either trying to download an update or your phone is flipping between 3G and 4G trying to lock on.