I left my iPod in the sun and the screen is darker?

I accidently left my iPod in the sun in the car today and it was pretty hot. Well when I got in the car and went to play it, I noticed I could just barely make out the screen on the iPod. Even if I set up the backlight. I also checked the contrast and that was fine. For some reason the screen itself seems to have gone darker. But it actually once I took it in, went back to normal again a few times then back to darker. What is wrong with this iPod?