It's natural, especially if you have been at it continuously for some time. I'm in FT work at present (quitting over the summer to start the PhD in the autumn) and feel drained and motivationless. It's probably just that your batteries are low and need refreshing (I assume you're still interested in your topic!).
How about taking a week or two's holiday, get well away from the university and do something active that involves nothing to do with your PhD.