Eventually, I figured out what was wrong. I was sick because of the altitude, but the symptoms weren't bad enough to be called altitude sickness. The mildest form of that is called acute mountain sickness, or AMS.
AMS is often experienced at elevations of 8,000 feet and higher. My hike ended yesterday at around 8,700 feet, but I know I didn't have AMS. Its symptoms are worse than what I had. They usually include headache and dizziness.
Nevertheless, I felt ill because I didn't take time to adapt to the altitude before I started hiking. What I felt was a somewhat new experience for me.
When I hiked the Pacific Crest Trail and the Continental Divide Trail, I gradually hiked to higher elevations. Those trails went much higher, 12,000 and 13,000 feet at times. The highest point of the CDT was 14,278 feet. I had been hiking several weeks before I got to that elevation.
Thinking about it now, I realize the only time I felt similar symptoms was during an attempt to hike the Colorado Trail in 2023. I felt nauseous then too, and like yesterday, I failed to recognize the reason.
Just before that hike, I picked up a bug at my grandson's birthday party. I wasn't feeling well when I boarded my plane to Denver. Later, when I started feeling nauseous, I thought I was still feeling bad because of a three-year-old’s germs.
Now, when I put the two incidents together, I realize they were the same.