Why does your body suddenly 'shiver' after holding your pee and then urinating all at once?

This is a dual reaction of the nervous system rapidly correcting blood pressure and heat loss! This phenomenon is medically known as 'Post-micturition Convulsion'. When you hold your urine, the expansion of your bladder stimulates the sympathetic nervous system, keeping your blood pressure and heart rate at a higher level. When you expel a large amount of warm urine all at once, two physical and physiological changes occur simultaneously: 1. Instantaneous Heat Loss: The rapid exit of fluid with body temperature (approx. 37°C) causes a slight drop in body temperature. 2. Sudden Blood Pressure Drop and Parasympathetic Nervous System Intervention: The bladder pressure is instantly released, and the parasympathetic nervous system immediately activates a 'blood pressure lowering' mechanism. To prevent dizziness from blood pressure dropping too quickly, the brain instantly switches to the sympathetic nervous system, releasing a small amount of epinephrine and instructing muscles to perform a rapid, minor contraction (shivering) to generate heat and pull blood pressure back up!