“Fainting after a postpartum bowel movement is an example of a ‘vasovagal episode’ or ‘reflex syncope,'” according to Dr. Jacobson. “Physiologically, this is due to failure in the auto-regulation of blood pressure, and ultimately, a drop in cerebral perfusion pressure resulting in a transient loss of consciousness.”
So basically, the lack of blood flow to the brain causes one to faint, due to the body’s inability to regulate its own blood pressure after giving birth.
Source: https://www.buzzfeed.com/michaelabramwell/pooping-after-giving-birth-tiktok