Is it possible you got a little water in there? I don't know where the iPhone 5's moisture indicator is, but on previous phones, it's in the headphone jack...look in there, if you see red, it mean's the phone's gotten wet. Maybe on the 5 it's in the lightning connector.
Maybe a shirt fiber? Were you wearing a red shirt around the time when you got the iPhone? I got a shirt fiber stuck in my N64 cartridge slot![]()