Here is some more on this...
Similarly, if we have a particularly large bowel movement the vagus nerve can
become over stimulated again, taking blood away from the brain to help with
what's happening elsewhere in the body. It's an unpleasant reaction and one that
can be embarrassing but medical professionals recognize it readily and know the
very simple treatments required for vagal nerve fainting like the backs of their
hands.
http://ibs.about.com/od/symptomsofib1/a/IBS-And-The-Vasovagal-Reflex.htm
Please don't be alarmed by the mention of fainting. I have had this sensation many times, and have never fainted...it just passes...so to speak! ;p ;p ;p
S.C.