My iPhone 5S is over heating when talking on the p...

My iPhone 5S is over heating when talking on the phone for 10 minutes, even with it's charging for 30 minutes. So today I brought my iPhone 5S to this Apple retail store, and they told me my phone is fine based on the diagnostic report. And what they meant is that I "came for trouble", they refuse to give me a replacement. They are expecting my phone to be shuts off by itself from over heating next time and then bring in the phone. I was so pissed, because this means that this phone might explode when it's over heated, and I have to live with panic every time when I'm using the phone or charging.