There could be an app running in the background that is stuck, or crashed. Sometimes my Map app, or a game running can cause my phone to heat up. When my phone heats up, I force close all the open apps, so that they all close and none are running. This always makes my phone cool again. Also, It's good to restart the phone every few days to keep it running smoothly. Or, if you are in an area with poor cell reception, like 1 bar, the phone could heat up as it transmits at higher power to the cell towers to maintain a connection. Or, if you recently installed the latest iOS update, your phone will heat up for a day or two while it re-indexes all the files, and updates the phone. Under Settings --> battery, you can see what app is draining your phone the most.