I have no experience with a mill pup doing this but have to say that it breaks my heart as this is all she has ever known.
How long have you had her? I am wondering is she is new to your home and only recently spayed maybe her hormone levels need to go down and she will stop? It is usually a sign on dominance and not lust that a dog will hump but with a pup that has lived in a mill making babies their entire life I would think it might be a little different. Maybe a way of coping
