Article by AppleToolBox
Several new iPhone 5 users have reported that their iPhone is getting abnormally hot. The new iPhone (iPhone 5) has some impressive features. When you are charging your battery or using your iPhone for a period of time, it is normal for the iPhone to get warm. If you use your device in extreme conditions with temperatures that are too low or high, your iPhone may not function properly. For instance, the iPhone may overheat because of weather conditions and the associated temperature (e.g. leaving your device out in the sun for a long period of time). This may lead to your device:
(a) not charging
(b) going black
(c) showing an alert message that your iPhone needs to cool down
(d) showing weak cellular signals
(e) disabling camera flash.
If you experience this, turn your iPhone off and keep it in a cool place for an extended period of time before attempting to use the device again.
If your device is overheating to the point that it really hurts when touched, contact Apple support ASAP; this probably indicates you have a faulty device.