I sometimes have weakness after bowel movement. it seems most pronounced after being constipated and then having a large bowel movement. ?

Vasovagal event: The vagus nerve is a cranial nerve and after exiting our brain follows the course of our gastrointestinal tract. It sends branches to th carotid arteries and to the heart. During a bowel movement one raises pressure in abdomen in effort to evacuate. This can raise blood pressure and stimulate carotid artery. A neural impulse can feed back to the heart and slow the heart and abruptly drop pressure.