The building announcement of the body that it's time to have a bowel movement is a stressing factor that requires cortisol.
Having the movement itself requires quite a lot of it.
Afterwards, when things have to shift back into place and new homeostasis has to be established, cortisol is required too.
So my theoretical answer to your question is: it's the increased demand on your adrenals (and their insufficiency to provide) that causes the jittery.
Here now follows my anecdotal experience. Context: I have adrenal malfunction and take up to 20 mcg Hydrocortison per day. My BP is still too low and I haven't got intestine motility without hydrocortison and lying flat ("supine"?).
I do not get jittery before a bowel movement.
I do need to take extra hydrocortison beforehand because during and afterwards I'll come down hard, adrenal wise. I may even crash. Often I need to lie down for an hour, in a darkened room, waiting for my body to sort things out.
I do feel enormous better when there's nothing in my bowels, however. Having stuff there is really tiresome to the system. If I didn't like eating and level blood sugar I'd opt for a fruitarian lifestyle or somesuch and live on fruit water and never have a bowel movement again.
Hope this perspective is helpful.