booked an genius bar appointment, tbh im in uk and i bought it from a carrier (not sim free at an apple store) so they wont be able to replace it till next week, or its dependent on wether my local apple store has them back in stock.
but everything works, and no scratches so I question if its worth replacing it ?! as some have stated this as normal, some have said it shouldnt, and also alot of different answers as to what it could be ..
mine is defo not the battery tho, that seems alot more serious and my phone works fine.
ill see what the genius bar say just to make sure.