No. Lilith is on the right track regarding cause. You're probably putting pressure on the vagus nerve, which causes your blood pressure to suddenly drop, depriving your brain of oxygen, causing you to get dizzy and/or actually faint. It may also be similar to the reaction of some people when they strain to have a bowel movement and also hold their breath at the same time (unknowingly), and then relax. It causes some weird changes in where your blood is going and can cause you to faint or worse. On that happy note, I'll shut up.

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