Hi Kipto,
I have exactly the same problem and I have been searching for others who have seen this too.
My wife & I have an iPhone 5 each, both purchased around DEC 2012 also in UK on O2 and BOTH phones do this.
I have read elsewhere and have been following the possiblity that this is caused by interference when the phone has switched to a 2G data network, which is represented by an "o" on the screen in the UK on O2. When the phone is showing this symbol AND your phone is trying to send or receive data AND you are using your camera (Movie or Photo) there is a high probability that you will see these pink interference lines on the screen. Sometimes it takes a while to appear, sometimes it appears straight away. If you are connected to WiFi or 3G, the interference seems to never appear. If you force the iPhone to connect via 2G by switching off 3G and Wifi in the settings, see what you get when you use your camera, it does need to send or receive data at the same time.
It appears that a really good work around in poor signal areas is to switch ON "Airplane Mode" when you want to use your camera, it works perfectly here. Another trick which I have been experimenting with is to double tap your home button and stop running every app other than the Camera app, this seems to work so far. This would reduce the data transmission on the 2G signal but Airplane Mode is the best.
I have also noticed that when the iPhone is connected to 2G and is either searching for a 3G or a WiFi connection or even sending data slowly, it raises its temperature, this may well be another sign that its working hard.
Somehow 2G causes problems with the camera when the iPhone is sending or receiving data but by making sure this does not happen, the camera seems to work with no problems.
I have done this on both iPhones 5's (1x Black 1x White) and they are both exactly the same so at the moment I am reluctant to exchange, lets see if iOS 7 makes this any better.
These are my findings so thought I would share.