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. **
Thanks. Will check on that.
I agree, it was most likely the vagus nerve. I have Vasovagal Syncope disorder, meaning I get this response a lot. Just being on my feet for a long time can cause me to black out. Cardiologists use the tilt table test to diagnose the disorder, but since it has rarely happened to you, that's not relevant to you. It most commonly happens during bowel movements, period cramps, when hit by sudden hot water (like a shower or hot tub), when forced to stand in one spot for a long time, or when doing something that takes exertion and causes you to sweat a lot. It is basically just a few seconds of your blood pressure tanking. Sitting down, putting your head between your knees, or laying down can help it go away if you feel it coming. Also, salt can help bring your bp back up when it happens.
This reaction becomes more likely if you are tired or dehydrated, so make sure if you're going to be somewhere that you're on your feet a lot, you get plenty of rest and water. If you're afraid it will happen again, I've found salt tablets help me if I need to bring my bp back up. Because having a vasovagal reaction can make you feel light headed and weird all day.
Hope this helps. Google will be able to tell you tons more about vagus reactions, and so can your doctor.