This is just a theory on my part, but I think extreme fatigue can follow a period of intense cleansing as a way for the body the shut down and rebuild. I have had this experience with different fasts and cleanses over the last few years. Even now dealing with a spinal injury that results in chronic pain and weakness, I find some days I just sleep the day away. If this continues too long, you might need to have it checked out by a health care professional (hopefully holistic or a Naturopath). However, if it only lasts a short time, I suspect it is part of the cleansing, healing, rebuilding cycle that will result in improved health.