Iphone 5s Bump on back when first received.

I received my Iphone 5s about 3 weeks ago. When I first received it, I noticed a little bump on the back. At the time I wasn't really concerned at the time and thought everything was fine. I took my phone out of the otterbox case today to clean it.I came to find that the bump is starting to cause the metal to crack or something. I can't think of the correct word. Did this happen with anyone else? Will apple replace the phone for free? Should I take it into my carrier, Best Buy, or an Apple Corporate store?