That happened to me last year. i don't do well in heat (and a humid heat that we get out here, at that) and went on a 45 minute ride with one bottle of gatorade. Majorly bonked, almost passed out, on one of the last hills. After that I installed a behind-the-seat double bottle cage and now carry 3 bottles on really hot rides.

I also try and load up on electrolyte drinks (Cytomax or Gatorade or some such) from lunchtime till when I ride so that I'm super hydrated and that seems to help.

K.