You would have dropped everything to run to the store. I'm sorry, but I'm in the middle of something. You're still doing it, but on your terms. If this person is a general pain in the neck and you really don't want to be bothered, don't offer to go later. Suggest they call someone else.
Guilt is merely manipulation used by selfish, self-serving people to get their own way.
Don't buy into it any longer. We're having meat loaf. You should have said something this morning. I'm sorry you feel that way. A very subtle, non-confrontational shift in power and control.