False pregnancy is a condition in which a female exhibits behavior that resembles pregnancy. This is caused by the influence of hormones produced by the ovaries. During false pregnancy, the female seems restless and tends to “mother” toys, shoes, or other articles. She may attempt to build a nest with torn paper or blankets. Her abdomen may seem larger than normal, and the mammary glands may actually produce milk.
All this is fairly common in dogs and cats. Both dogs and cats have fairly high levels of progesterone for a couple of months after heat whether they get pregnant or not and if the progesterone level is high enough, it will stimulate uterine enlargement, mammary enlargement and production etc.  Progesterone is the hormone that human pregnancy test kits detect, but in dogs and cats these tests are unreliable because progesterone levels are often high even if not pregnant.

Treatment
In most animals, no treatment is necessary, because the affected female will “cycle” out of the false pregnancy on her own. This usually takes 2 months; the same as a normal pregnancy in cats and dogs.

Surgical removal of the ovaries and uterus (spaying) may be advisable in females that have severe or repeated false pregnancies.

If your pet is especially uncomfortable, you can provide some relief with warm compresses, bag balm, and benadryl, Rimadyl, or similar products.

Your vet may also prescribe some steroid anti-inflammatories if needed.

The big worry here is either uterine infection and/or mastitis. See above articles and comments about these serious problems.