Welcome to Nikonians! Are you using a hotshoe bubble level ? Have you tried using a different lens and if so, did the problem persist? If you are using a hotshoe bubble level and using another lens doesn't fix the problem, I would suspect the sensor is not aligned correctly. If you are determining level by looking through the viewfinder it is possible that the focusing screen retaining wire came loose. Here is a link to instructions for changing the focusing screen on your D5100: www.katzeyeoptics.com/files/install_13.pdf If the screen is loose you should be able to tell if you take off the lens and look closely at the finder screen at the bottom of the mirror box. Good Luck and Enjoy your Nikons!