A tip regarding iphone overheating if you updated to ios 8.3 using your phone instead of itunes.
I updated my iPhone 6 via downloading the update over the phone instead of using iTunes. My iPhone 6 starting to run hot the day after I updated. I wasn't even using it.
i decided to wipe the phone and completely do a full restore from itunes. My iPhone 6 has been running cool like it should now.
iPhone 6, iOS 8.3