I'm with JP on this one, fatigue, dehydration and heat. I had a friend collapse on the street just the same way, the paramedics thought he was having a heart attack, but when they got him to the hospital figured it was just dehydration and set him up with an IV, and he was fine a couple of hours later and he wasn't even doing anything physical. Ya gotta watch your fluids, it'll sneak right up on ya.
Glad it all turned out ok!
love,
Ziffa