As the waste is washed away the draining of it slows you down. You are often bloated just before this happens & if you suffer cramps back aches etc that will wear your energy down as your pain tolerance is being tested also.
You might have an iron deficiency. Make sure you take a multivitamin such as Centrum on a daily basis. Also start taking Vitamin B or B-Complex. B vitamins are natural energy boosters and stress relievers.