I did the same thing with my medium size cutting mat that I was taking with me on a weekend trip to the beach. It warped in the car from the sun & molded to the shape of whatever was under it. I still use it but I could never get it to be completely flat again. I tried ironing it a little & that helped a little. I thought about putting it on something very flat & out in the sun again but never got around to it. Let me know if you ever figure out how to get it completely flat again!
Pam