This is a strange one, I have not heard of this before, phantoms ovbviously, and I am of the mind to thinking that she is having a mild phantom. Well, logically, I would think that she will come in later rather than earlier, if her body is producing milk, then her body thinks she is having a litter, or feeding a litter, so I would think whilst producing milk her season will not start.
If she should come into season, then her body is ready for pups, with the same chances of pregnancy as any other bitch.
All you can do is wait and see what her body decides to do.