I can relate. We had a similar "on" switch with the hot weather in CA, when it shot up to 95 all of a sudden. I can't just go out and ride in that without any adjustment. It drives my heart rate up, and I end up with a headache if I'm not careful. I get really red in the face. I'd scare children.

From what others have said, drink a lot of water. I think it also helps to be hydrated well during the day and possibly the day before.

But adding hills with the hot weather... might be asking for too much.