I also feel very tired all over for hours after a bowel movement. Not after every BM, but after a lot of them. I have to lie down. Sitting down is not enough of a rest, and a nap is preferable to just lying down. The whole-body fatigue is joined by an ache in my legs. This tiredness can last for a few hours or all day, with the potential for lesser but lingering tiredness the next day.
I don't need to strain when I have a BM, so it might not be the vagus nerve being stimulated.
I have comments about two potential ways to lessen the fatigue.
(1) I have been prescribed a non-addictive muscle relaxant to take an hour before bedtime called cyclobenzaprine, and it helps a lot, but not completely. My doctor said that if a constricted nerve in the lower back is an issue, then that can cause leg aching and an overall tired feeling. I have had a minor lower back injury, so this can make some sense.
(2) There is another thread on ehealthforum that covers fatigue after bowel movements and in it one person said this fatigue no longer occurred after being treated for a D. fragilis infection. The thread is called 'Fatigue After Bowel Movement.' It turns out that D. fragilis is a parasite that can be in the large intestine of people and some other animals. According to various Internet medical sites, this parasite might cause no symptoms or can cause gastric pain and other symptoms, including fatigue. For a while, the medical community thought that D. fragilis was a benign, inconsequential parasite with no ill health effects. However, it apparently does have ill effects for some. There are effective treatments for this parasite. A bowel parasite would seem to make sense for symptoms after a bowel movement, would it not? I'm glad someone found complete relief, and I'm also glad we have another avenue to pursue.