It's a bad idea to post your serial number in a public forum... however, since you did so, I checked at https://selfsolve.apple.com/wcResults.do
It's covered under warranty until June 19th. Since it has a 1 year warranty, that means the first sale date was in June of 2012.
I don't know where you got it, but I have to assume it was a gray market reseller. The phone is definitely NOT new. There's no telling what it's been throuh between the time it was originally sold and the time you bought it.
Make an appointment at the genius bar of your local Apple Store and have them look at it. It's still covered and wll be replaced if it's defective. If it was hacked, jailbroken, or modified, though, they will turn you away.