
I had a Kerry Blue bitch years ago who did this - the vet reckoned it was a sub-involution of the placental sites. She was fine in herself, and it cleared up after the next season. Interestingly, a couple of years later she did, in fact, have an open pyo, but nothing to do with any whelping - she'd had no pups in the intervening time.
Maybe the two were somehow related, I don't know...
Jo and the Casblaidd Flatcoats