Now We Know...
... My exhaustion is the altitude. All the websites I read say that altitude sickness kicks in at 8,000 feet. At an average, the altitude in both Flagstaff and the Grand Canyon is 7,000 feet. But we dropped down about 2,000 feet to go to the Wupatki National Monument and I was fine.
Good thing, because the pueblos are amazing! We had some hiking and climbing and crawling to do. Some of those pueblo doorways are pretty short.
As Ted White, the ranger and Japanese Language Interpreter at Wukoki ruins told us, "It's pretty amazing to stand in an 800-year-old building and know that it was probably the tallest structure in the United States for hundreds of years."
I was also impressed with the Nimbus 2000 he was using to sweep away the footprints of all the naughty people who went off the path.
But now we're back in the hotel in Flagstaff, and I get out of breath bending down to pick up a cat. The less-than-helpful altitude sickness websites also tell me that the only real cure is to go to a lower altitude.
That's not really an option. I am going to the Grand Canyon tomorrow. So instead, I guess I'll take the other bits of advice: don't exert, drink plenty of fluids, and eat carbohydrates. I guess it's not all bad. I love it when I have an excuse to be lazy.