Blank screen on 5.1.1 - N910C

Updated N910C to 5.1.1 few weeks ago, it was running all fine but for the past couple of days I am noticing this weird behaviour where the phone hangs for a while and shows the blank screen (wallpaper) for a second and then the phone is locked. When you unlock it you're shown the screen you were on before.
Can anyone confirm if they have the same issue? Any suggestions would be appreciated too. Thanks!