
As long as you are 100% sure she can't be pregnant then ignore the situation,don't be tempted to squeeze nipples as that will cause more milk to be produced rather than it drying up, don't show sympathy or she will drag it out and may even go as far as lactating and labour,take any objects away that she may be "mothering",soft toys etc., the longest my girl lasted was 7 weeks and then returned to normal literally overnight.
All bitches have phantoms to some degree as in the wild they would help to rear other pups by producing milk.