iPhone 6s gets really hot often

I've had my iPhone 6s for over a year now, and it gets hot whenever I use it, no matter what apps or what I am doing on it, then I stop using it & it cools down. When I purchased it last year around August, after a few months it got to the point that I could't use it due to a warning saying: Overheat. It wouldn't stay on, so Apple replaced it. Now I am worried the replacement iPhone 6s will do the same and I am past my 1 year complimentary warranty. I have called Apple Support, gone to the Genius Bar, and they don't see anything wrong with it. I just believe the 6s was poorly made, I had a 5 before and it work perfectly for years, and I have almost the same apps now. Does anyone have the same problem?