Hi! Google, or search for the Vagus nerve response. It can happen during
bowel movements, stressful bowel activities, and at other times as well. I have a friend who is a RN and she has told me that they have actually had people pass out and fall off the toilet. Sweating can also be involved. I have had the same thing happen to me. I have never actually passed out, but it's an awful feeling. Good luck and hope this helps.