Flash poopy

When you leave your house feeling fine and you get to your destination, but all of a sudden you need to defecate (poo)... now.

Much like a Flash flood, except coming from the sphincter, and not the rain.

Martha: "I had a really great date, do you want to come inside? ;)"
Gunther: "Sorry Martha, I just got a crazy flash poopy going on here. Raincheck?"