Why does my Nexus 5 keeps switching itself off?

After some minutes of non-use, my Nexus 5 goes to "sleep" i.e. screen turns dark. If I feel the need to use it, and I press the power button, as soon as I enter my password or unlock pattern, it switches itself off, then proceeds to loop, going off and back on by itself.
This carries on until I press the power button for abit.

This issue started about 3 days ago. Anyone else experiencing the same or have a solution?