I would check your temps, make sure they are correct, then get the snake to a vet. It sounds like a respiratory infection at the very least.

Maybe just take the snake to the vet first, but without a thermometer there is no way to tell if your temps are correct, just becuase its hot outside does not mean the temps are proper in the snakes cage. I live in Phoenix Arizona and it can hit 120F in the summer and I still need to keep an eye on my temps.