That is strange. I'm not sure why it's happening, then. What does your doc say? Could it be a physical reason, like your thyroid? Lithium can exacerbate thyroid problems. It didn't do it to me, but after being on Lithium for just over a year my kidneys quit filtering it and I kept getting toxic, so I was taken off the lithium quickly and put on Abilify. That probably made the fatigue worse in my case.