My vote would be dehydration. Heat really zaps your strength, I was a lifeguard in the summer and people laughed at me when I got off and was exhausted some days, they said I just sat around all day, and while true, I was in direct sun and heat for long periods of time. I know when spring and summer rolls in around here, 100 plus days, in West Texas it takes a week or so for your body to adjust being out in the heat all day.