Question [Q] [DISCUSSION] Does anyone's Xperia S's Back Cover Smell Like Melting Plastic?

So I've had my Xperia S for almost 2 years now and in the past year, I've noticed that the back cover of my XS smells like burning or melting plastic. I've always had a plastic hardcover case on it (spigen) and occasionally/rarely I use my phone to the point where I can feel the heat.

Does anyone else have this problem?