The stain could be from the fixative or even the quality of paper you use and the scanner/camera. Flash bulbs can lend lighting and staining effects to paper that we can't always see with the naked eye. I had some pieces that had that effect becuase of some white-out used to correct some bad inks. Thank god for digital editing lol! Then again, keep in mind that your hand can secrete oils that might not be visible either, especially if you are doing a detailed piece and sweating some from your hand resting for too long, etc.
Hope that helps.
PS Oh yeah, great rendering, pencils like that can be a pain if the graphite (or lead-susbitute lol) smears. Good shading and textures.