I have heard of urination syncope, where the vagal nerve (10th cranial nerve) gets over stimulated during urination and causes
reduced heart rate, sweating, flushing, nausea, and even fainting
(a vasovagal response can be triggered by several different things...take a look at Wikipedia's page on Vasovagal Response.) also, you might look up urination syncope... It is usually benign, but it can also have an underlying medical condition that needs to be looked at.