But I have an iPhone 6 w/64GB and all is well. It wasn't new but a refurb for just about 200USD so it was an upgrade from my very old still working i5s.
In your case I would at the very least turn it off and on again (reboot too.)
I am using an Iphone 6 (64gb) one day while browsing the internet the phone started to heat up and got stuck suddenly after that I started to restart many times it want working meantime I got a call and it was ringing and I swipe right i can attend the call. I don't know what happen to it and what can be the problem?