On the serious NOTE
why is it that when you take a big dump you get light headed?
Vagus response. You Push Push Push on your vagus nerve then you get up and it causes a severe case of orthostatic hypotension.
I dont know, educated guess, I guess..?
I seriously doubt it's the vagus, because the anal sphincter's under somatic control (and for a good reason!).
Maybe you're trying so hard with your abdominal press that you just forget to breath?