Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
If the cartridge you put in was second-rate (i.e. 'cheap', inexpensively produced or remanufactured or refilled or 'Canon-compatible'), suspect a defective cartridge. Especially if the problem did not show up just before it was replaced, perhaps a different problem then but not grainy copies.
If the graininess was already there before the cartridge change, then you might unfortunately have a problem with the copier itself not the cartridge(s). So please describe in more detail what the grainy copies look like... or, scan an example and post it to your Reply, so that I can see what it is actually like and be sure to advise you further in a fully appropriate manner. Thanks.