If you have not already I would get your thyroid checked. Lithium messes with thyroid function and slow down motabolism. I had to go on a thyroid hormone replacement due to the lithium. Don't mess around with low thyroid because you can go into acoma if it gets too low.

Signs of low thyroid that I can remember:
tired all the time
hot and cold flashes
body aches.

I know there are more, but I can't remember. The tired all the time is the main one.