That was also a week he spent travelling. Sleeping away from home, long plane/car rides, irregular schedule, and all the other attendant discomforts are quite enough to throw me off my game, even without dietary shifts.
Could also be a temporary effect. Your gut flora adjusts to what you're eating, and a sudden shift in composition can cause digestive distress.
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.