You are not alone! This is a common occurrence in dogs that have a long sixty di-oestrus phase after they ovulate, when progesterone, the pregnancy hormone, is high, whether or not they are pregnant.
The phenomenon stems from wolf packs where the dominant female only would be mated by many dogs (all of whom would think they were fathers).
All the females however would have this ?false pregnancy? and actually lactate so that the offspring could be fed, not only by the mother, but also by numerous pack ?aunties?.
With all the females involved in mothering and all the males thinking they were dads, optimum survival was achieved for the young.
In pet and working dogs, however, this milk production is a nuisance and can lead to mastitis.
An effective treatment called Galastop is available from your vet.
Your bitch may also be at a higher risk of pyometra, which should be checked out.