Share your experience!
You need to either return your device to the place of purchase or contact Xperia care http://www.sonymobile.com/global-en/support/contact-us/
Hard to say what the cause can be but if the phone has been in contact with water, this may have damaged the phone. I would suggest the same thing as Thommo if you experience or begin to experience abnormal behaviour, to contact your local support team.
Some accident out the bedroom window