You were smart to realize something was wrong and get some food & water. It just takes a few days (or more) to adjust to heat. I struggle with it every year.

I finished Bike Across Kansas on Saturday, 60-80 miles daily. I tried to go for a short ride Monday evening but it was too hot and I didn't do well. (I nearly threw up.) It was unusually cool for Kansas last week. Then on Tuesday I biked to work, not even 2 miles, and got super nauseous again. Wed-- I was feeling sick before I even got on my bike. (I biked anyway. Slowly.)

Heat and fatigue, I guess. I'm off to NIH in a couple hours (by plane, not bike!) so no more biking this week. Hopefully next week will go better.