If you were getting chills at your first stop, you were ALREADY experiencing the beginnings of heat exhaustion. You're lucky you didn't have more serious consequences.

Never ignore chills. Stop, get in a cool bath if one's available, or soak your clothes and cover yourself with damp paper towels if you can't take a bath. Drink ice water. Call someone to come pick you up. If you absolutely must continue riding (must, as in, everyone you know with a car is in the hospital and there's no public transit near you), then wait at least an hour for your core temperature to come down, keep replacing fluids and electrolytes, wait until a cooler part of the day if that's possible, and then return SLOWLY.

You didn't say what kind of conditions you are acclimated to, but if your weather has been anything like ours, you're probably not acclimated to the heat at all. It takes some time, at least a couple of weeks of consistent exercise in the heat. This time of year in most places - and it seems especially acute this year - the temperature is so variable that it's hard to get acclimated to anything.

It also doesn't sound like you were replacing enough electrolytes, if all you had was one diet Gatorade. You didn't say how much water was in your Camelbak. Everyone's different and it takes trial and error, but it's something to be aware of.

Finally - the most minor point, but it may have contributed to your cramps - diet drinks don't sit well with a lot of people, particularly during exercise.