I'd put my money on one of 2 things - the last app you installed or a 4.4.2 problem.
First try a safe mode boot. (Power off, power on, then hold the Volume Down button until it gets to the lock screen. You'll see "SAFE MODE" in the lower left corner. If that fixes it, it's an app. (Safe mode loads the original phone, but nothing that you've installed.) You get out of safe mode by just rebooting normally (long press the power button, restart).
If not, make sure everything is backed up to your computer, then do a factory reset. Restore a few apps and see if it's fixed. If it is, restore everything else. If it's still not fixed, being it in and see if they can do something for you, but you'll probably walk out with a replacement phone.