why does my iPhone 5s get hot when i use it?

Why does my iPhone 5S get hot when I use it sometimes? This is an intermittent problem, not always occurring when charging, just in normal use. Has latest iOS 8.2 downloaded, but problem has occurred since new. The phone gets to hot to touch the body. Phone is 14 months old. Is this dangerous? I know that batteries heating up can result in them exploding. Have tried turning off, and removing cover - no longer use a wallet style cover because of the problem. It is intermittent but concerning when it happens.