If it was the helmet, now I think I know what happened. We just went on a trip and the helmet had been bouncing all around in the car for days. When I put it on this morning, one of the snap things on the side had come undone and I had to re-snap it. I bet I accidentally cinched it up too much. At any rate, I'll keep an eye (or a brain) on this and see how it goes. Thanks for all the helpful input.