Nikon lenses have a mechanical connection for aperture control that is spring loaded at both the camera end and inside the lens that could suffer damage if disconnected while in the wrong position. If the tab on the control lever in the camera is bent it causes incorrect aperture settings that usually result in overexposure (because the tab usually bends up and the lens doesn't stop down as far as the camera tells it to stop down). The linkage lever inside the camera can also be bent by improperly attaching a lens. In some cases it can fail to engage the aperture control lever on the lens at all!

One way to test for this is to set the lens to the widest aperture (lowest f-number) and see if exposure is affected the same way as if the les is set to the minimum aperture (with a corresponding shift in shutter speed to compensate). Another way is to use the Depth of Field Preview button (DoF) to see if the lens stops down any from the widest aperture when set to a stop or so narrower. If you are using the 18-55mm kit lens zoom it to 55mm (because the extra magnification helps you see the aperture by looking down the front of the lens) and set the aperture to f/5.6. Then press the DoF button to see if the lens stops down any. If not, set the aperture to f/6.3 and observe if the aperture stops down when the DoF button is pressed. Progressively set the aperture value in 1/3 stops to f/7.1, f/8, f/9, f/10, f/11, f/13, etc. and observe if the the aperture stops down when the DoF button is pressed. If the aperture isn't stopping down until you've gone a few stops into the series, then you have a problem with the mechanical linkage between your camera and lens.