In the old days they'd say that and really use the time to insert birth control if it looked like it was headed that way...
I wouldn't be surprised if it was a general physical reaction to getting stoked down there.
And I've heard where women lose bladder control if they're laughing too hard or getting stimulated beyond what they can handle--(something of compliment in there, I think...)
Or they simply need to go more often (aren't they always running off to the restrooms with their girlfriends???)
So write these out on a card and next time this happens, have her check off the real reason. Compile your results. Let us know. God knows enough doctoral quacks have gotten research grants doing sillier work before...