Looks like a processing fault but can't be sure, if it were damage to the emulsion within the camera you would expect to see it on all of the frames.

Your image does not look grainy to me. There is no connection, that I am aware of, between the use of a CD and the graininess of an image. Typically you get noticeable grain if you use a fast film, push the development, or scan with the sharpness turned up too much.