iphone 4s is getting very hot just after 5 minutes of Camera uses

My iphone4s is getting very hot just after 5 minutes of camera uses. Even I went to Apple store and my phone is replaced 2 TIMES (yes two times) with in a week, but both the REPLACED phones is giving heat problem. My Original iphone 4s never gave me any problem of heating, but gave ringer volume issue. So, Apple store replaced my phone on 9/22/12. But that replacced phone gave me heating problem of camera. Then I went again on 9/29/12 for this and the 2nd replaced phone is ALSO GIVING SAME HEATING PROBLEM OF CAMERA. I have shown this to genious bar person for the 2nd phone which was just given to me and there was no apps on phone. Still it got hot. Then genious bar person said, it is NORMAL to get heat in 5 mins of camera OR IT COULD BE DUE TO IOS6. (because my original phone was on 5.1.1 and never gave me any heat problem). So, my question is anybody is facing problem of heat during camera use specially after upgrading IOS6? Or it is due to device and I need 3rd replaced phone? How can I downgrade to 5.1.1 so taht I can check is it due to IOS6 or device only?

Please suggest me asap as warrenty of my phone is going to expire very soon. Any comment is welcome. Thanks