Flash is Disabled - The iPhone needs to cool down before you can use the flash

I've had my iPhone 6 for a little over a month. Yesterday I updated to iOS8.4 and today, for the first time, I noticed the above message when I was taking a photo. I didn't need to use the flash but I'm concerned about the message because the iPhone was cool to the touch and I'd only taken two photos with it today.

I Googled the error message and saw various suggestions as to what causes the iPhone to overheat, however, none of these applied to my phone and, as I said, it certainly didn't feel hot.

Also, I've been inside most of the day and the temperature inside is 25 degrees Celsius. I've taken off the Apple protective silicon cover for the iPhone but I don't really believe that Apple would sell a case that would cause the phone to overheat. The triangle with the exclamation mark is still showing when I go to the Camera app. and it has now been there for a couple of hours.

It's just after 8pm here now. Obviously, I won't be using the phone at all overnight. If the message is still showing in the morning should I assume that this is a fault with the hardware/iOS and take the phone back to Apple?

Any suggestions as to how I can test the 'real temperature' of the iPhone 6 would be appreciated.

Cheers

Tricia