It is explained as follows:
1. Vasovagal reactions occur when you stimulate the vagus nerve (vagus comes from the Latin "to wander") - a big nerve that "wanders" all over your body. Vasovagal reactions are the most common cause of fainting.
2. There's several ways to induce a vasovagal reaction, and having a bowel movement accomplishes 2 of them!
3. First, any kind of stomach upset can cause it, because the digestive tract is ennervated by the vagus nerve (hence it is also called the pneumoGASTRIC nerve).
4. Second, holding one's breath and bearing down (like we do when we pass a poop, especially a hard one) is called a valsalva maneouver, and this stimulates the vagus nerve BIG TIME. Some people pass out (or nearly) every time they poop. Bummer!
5. So, your gastrointestinal upset +/- holding one's breath and bearing down are both adequate to stimulate the vagus nerve which causes an immediate drop in blood pressure and heart rate.
6. The result? A drop in blood supply everywhere - including the brain - which causes a sudden loss of consciousness. Once our head is lower down (eg. on the bathroom floor!) it's easier for blood to get there thanks to gravity, and we regain consciousness.
7. As for the diarrhea and cramps, lots of people have some combination of constipation and diarrhea - they strain to pass a hard stool, and then have diarrhea a short time later. When you're constipated, you often suffer the trademark excruciating cramps in an attempt to "move things along."
8. Then, once the "obstacle" has passed, you'll have a bout of diarrhea which results from the colon secreting fluid in an attempt to soften and pass the "obstruction." But it's the straining part that usually causing the lightheadedness or actual fainting.
9. Interestingly, fear can also stimulate the vagus nerve, as can pain. Hence, people who pass out when having blood taken.