Ph.D. work seems like a drag sometimes, is this normal?
This is quite normal - it is possible to be passionate about the topic, but at times, to be not so enthusiastic about the work involved.  This could be a sign that you are need of a bit of a break, or in the worse case, heading towards a burnout.

A few things to reflect on:


Are you getting enough sleep, exercise and eating well?
Are you making time to visit and go out with family, friends and other like-minded academics?
Are you giving yourself some time out to pursue other interests and hobbies?
There would have been times that the greats of the topic felt the same.


These greats of the field more than likely had days when they also could not be bothered.

Specifically, why not consider the following:


The days when the mathematics is very difficult, take these as challenges (rather than hurdles) to accomplish (rather than just overcome)
Teaching the class could be opportunities to refine your mathematical (and mathematical communication) skills and similar type of skills.
Is there anything you can change to make your situation more enjoyable?


It is very normal.