Why Do I Feel Tired And Lack Of Energy?

I have been going through boughts of being tired, lack of energy for a few months. I had a week of rest which seemed to help. Not sure what could be causing this. I get some exercise, eat relatively well. I am a school teacher and working all day and have a lot of papers to grade. I have never experiences this however. I can't seem to stay energized. I wonder if it could be nutritional issues? There are really no symptoms besides the fatigue.