Does anyone have this symptom?

Background: I 'cured' my IBS by going on a grain free diet (in 3 months) BUT I paid dearly for that diet...I became very sick because I became 'dangerously low' on salt and chloride... Now am recuperating but having a few old 'ibs problems'.
One of them, however is new to me (over the last month or so)
Not always--but OFTEN--after I have a large BM (no straining problems) I get unbelievably TIRED...to the point where it is hard to walk across the room. Sometimes feel a little 'sick' too but mostly this miserably disabling fatigue. It usually lasts 4 to 6 hrs and then still feel weak and worn out.
DO ANY OF YOU have this same symptom?? I would really appreciate your telling me about it.
I HAVE gone to the dr but he has no answers. Have any of you been told that this horrible fatigue symptom could be triggered by the vagus nerve?
I hope that eventually this will just 'go away'...but how I wish I could do something to help it go away.
THANK YOU to whoever answers. I would really appreciate hearing from you.

This symptom occurs when I have multiple BM's in a short time, the usual IBS result of trigger food consumption. My theory is a drop in blood pressure causing fatigue or it could be dehydration.

I drink a couple glasses of water over an hour and the fatigue goes away.

I also get a tired feeling from constipation and again drinking water is my remedy, and not eating anything with insoluble fiber.