Cracked Screen

I got my phone (Xperia Z ultra) in January 15, 2015 as replacement phone through J Cellbar. Today when i was arriving at work I noticed that my back screen has crack. This comes as a surprise to me because the phone has never been dropped and there was no pressure app**** to. When I purchased this phone i was under the impression that its a tough phone but i have doubts now since it just cracked on its own. I'm also not finding comfort in the fact that when I tried to reach Sony Mobile Communications on the number provided on the yellow pages they referred me to SSS Cellular. I was going through previous complaints people have with regards to the work these company performs and i was hoping that when you phone sony your problem will be handled not passed on a third party company. I need assistance on the action to take since I want my phone to be fixed and Sony should do it for free because the damage is not through my own doing.