I've only had this sort of happen to me once, it was caused by me forgetting I'd had codeine and drinking a can of Stella, then going for a piss, holy shit it was like I was draining all the life out of me, combined with someone kicking me square in the nuts, only without any of the nut-related pain, I've never known anything like it. I knew full well that the quick solution to codeine nausea is to lie on the floor for a moment until it passes, but the half-minute of piss I still had inside me took preference, it was a fighting battle to stay upright until I'd finished. Loads of fun. Wouldn't recommend it though.
My mate did once faint and smash his mouth open on the cat litter tray, so count yourself lucky it was your own piss.