Veronica,
It's just really difficult to get hydrated when it's that hot. If you're not used to it, I'm not sure it's possible to drink enough. I had a similar experience last Sunday. I didn't notice my heart rate climbing, but my symptoms were a slight headache and feeling like my brain was stuck in slow motion. It took me two hours to feel normal after I got home.

If you consider you're out there riding on top of reflected heat, with a plastic thing on your head, wearing thin clothing... I imagine that a long ride could be done, but it might require long stretches of sitting in the shade guzzling water.

I really hate this heat, it's 80 degrees at 7:00 in the morning.
-Amy