This is a normal part of quitting @Pilgrim. Just when you think you should be over the worst of the cravings, the nicodemon will rise up and bite you in the ass! You're denying your addiction and it doesn't like it. It's the final stages of withdrawal where those heavy cravings will hit you unexpectedly and they make no sense.
As others have said ... get busy; distract yourself. IT WILL PASS! We all experience some form of these sneak attacks later on in the quit process. I remember mine about 6 weeks after quitting. Came really close to throwing in the towel but knew I had too much to lose by them so just gritted it out. Time to bear down and get past it!