Thanks for the replies. It was definitely unlike any turbulence I'd ever experienced, but I don't think I've ever experienced clear air turbulence before. The weather was clear that day, so definitely not lightning. I guess the closest parallel I can draw is what might happen if you were in a powerboat and hit a small sandbar on the bottom which was small enough to keep the boat on-course, but big enough to make a good thump, and to make your heart skip a beat. It was there and gone, just like that. My wife was a little panicked, but I was mainly just curious as to what it was.