Hello, my name is Dr. Kara and I have over 20 years of experience as a veterinarian. I'm sorry to hear that Rosie is behaving strangely. I think that she is experiencing a false pregnancy.
This can occur after a normal heat cycle, even without a male being anywhere near her. This is because hormonal changes in a dog are very similar whether they are bred or not. In a pregnant dog the female's hormones fall off and the pups respond with their own hormones which leads to delivery.In a nonpregnant dog there just isn't any pup stimulation. But some dog's bodies get confused and they feel like they are pregnant.They will often pick a toy to mother, may even build a nest and produce milk.
We do know that if a bitch's mother had false pregnancies then she is likely to have them as well. And once they have one false pregnancy they are likely to have one with every heat cycle where a successful breeding doesn't take place.
It doesn't mean that she isn't fertile or that there are hormonal problems with her. She could be successfully bred on her next heat if you wanted to.
If you aren't going to breed her then I do recommend spaying her to avoid subsequent false pregnancies.
However there really isn't anything that you can do now to change her behavior now. Even spaying her at this point won't change things now as the hormones already produced and circulating in her body are the problem. Her behavior will change naturally as her hormone levels decrease. You can certainly try and distract her with walks and play as some bitches do seem to come out of it a little quicker if we can distract them. But spaying her after this false pregnancy will prevent further episodes.
Please let me know if you have any further questions.