My gf had the same problem with old iPhone 4. She went to the shop to try to replace it under warranty, but they refused to do that. They said something external caused it (like some liquid).
I know this is not helpful post...just sharing
. I hope that you will be able to exchange it for new one...
Sent from my Nexus 5 using Tapatalk
~LGE Nexus 5 / Asus Nexus 7~ ROM: Stock KitKat
HTC One X, Samsung Nexus S, HTC Desire, Nokia N97 (black), Nokia 6220c, Nokia E65, SE Xperia X1, Nokia N96, Iphone 3G 8GB, SE P1i, SE W660i, Nokia 7650 (first symbian s60 phone), SE T300i, Motorola V525, Siemens S45i, Nokia 8210, Ericsson T28, Ericsson R320, Nokia 7110 (first wap phone), Nokia 3210, Panasonic GD30...