Is there any possibility that it could be incontinence due to being spayed.
I have a similar problem with my pup and this is what my vet thinks it might be, although she only has 1 or 2 accidents a week when she sleeps so I haven't taken drastic action just yet in hopes she'll outgrow it.
It might be worth mentioning to the vet. I am told it is not that uncommon and can be easily treated with estrogen or other very affordable drugs ($15-20) month.
If it is dripping out while she walks and she doesn't appear in very much discomfort, it could be a real possibility.
Good luck with whatever it might be. I know it's frustrating.