I agree with Ariadne. Yoga may seem gentle, but as with any exercise, you can always overdo it. If you do not allow your muscles to recover between workouts, or if you conduct marathon-worthy yoga sessions, your body may cry out for help. Overtraining syndrome comes from exercising too much and is not to be taken lightly; it increases your risk of injury, lowers immune system capability and can even cause depression. If you feel severely run down, consult a physician for proper treatment.
Hope this helps