The appropriate response to the rude question is to look surprised, start spinning around try to look at your back and shout with fear in your eyes, "Holy crap! There's something wrong with my back?" Then stop, look the person straight in the eye and ask, "What's wrong with your manners?"

BTW, you're not the only one to have trainer mishaps. One of my favorite threads: http://www.bikeforums.net/showthread.php?t=240434 While there weren't others around to witness it, falling off, even on a stationary ride, happens.