Well congratulations on the week!! You are well on the way back to sanity.
I have done just the same thing, and left myself wondering "What the heck?" When things were going so well, some weird impulse led me to the liquor store. I can remember smiling as I loaded the (heavy-laden) bag into the back of the car. Here goes nothing! Literally.
I did some reading about relapse prevention and found several good things. The common point is that a relapse grows out of some emotional and physical work not getting done, and that it begins to be prepared long before that moment when you park at the liquor store. It begins to prepare itself when you first start to perceive yourself getting a little jumpy, frazzled, blaming, short-tempered, poor nutrition, no exercise, nagging worries, and don't do anything about these things but let them slide. Tick tock.
Best wishes to you for a solid return to health and happiness!