My Iphone 4s screen cracked by itself!!!

Hello there,

I went to a dinner tonight and my Iphone was in my bag. I took it out and took some pictures it was absolutely fine. I put the phone back in bag and after coming home I took it out and the screen was all cracked!!! I couldnt believe on my eyes. Without any reason how the screen can be like this!! I didnt drop it anywhere and there was nothing in my bag that can hit it. Also my phone was in a cover and a shield on the screen. Its really really weird.

I'm so much disappointing after buying so much expensive product and getting this result. I still can't understand what happened to the screen.

Anyone any idea or comments?