Have you seen this?

Then you may be using your iPhone outside its recommended operating temperatures. Apple has put up a few handy hints to keep your iPhone from getting too flustered.

 

Apple said:
Some conditions and activities that may activate the Temperature warning message:

  • Leaving the device in a car on a hot day.
  • Leaving it in direct sunlight for extended amounts of time.
  • Using certain applications in hot conditions or direct sunlight for long periods of time, such as GPS tracking in a car on a sunny day or listening to music while in direct sunlight.