There is likely food waste inside the grinding chamber that is starting to rot. It may be that you didn’t grind your last batch long enough, or perhaps there is food stuck inside the unit or not flushed down the drain properly.
To eliminate these odors, you should run plenty of cold water into the unit and turn it on. Allow it to fully grind and flush all items inside the chamber. Next, you should thoroughly clean your disposal to eliminate all bacteria that may be lingering and contributing to the odor.