It might be a simple slipped alignment of the focus ring.
But before you alter things you want to validate what the focus is currently. Here we want the lens to be at full aperture (f/2.8) so the depth of field is the narrowest, alter the lighting and shutter speed so you are at a minimum of 250 or faster so vibration is not introduced and still have a good exposure.
Now with the camera on a tripod and a simple B&W hunting type of target spaced out from the camera using a tape measure check 20, 10 & 5 feet or if you are metric 6, 3, 1.5 meters, measured from the film plane (CCD sensor) with the target in the exact center and parallel as well. These should be marked on the focus ring if yours are different alter the test to match.
Using the cameras auto focus does it line up? Then check manually. I would even make a few prints to compare. Depending on what you discover you may need to adjust the set screws which sit under the ring covers. If the settings are correct you may need to get a different lens if you are trying to do Macro Photography.