I've had ME for almost 10 years now but this has only been happening for about the last few years. Basically what happens is around 20 minutes or so after a bowel movement I will begin to start feeling very tired and weak, to the point where I will have to write off the next few hours and just sit/lie down. After several hours I return to my normal level of functioning. I don't have a bowel movement every day. Is this some sort of PEM? I suspect it's not, because it hits me quickly and it's gone in a few hours. If it's not PEM what could it be?