This usually lasts about two to three weeks, which coincides with the times from being in season to a couple of days after when she would have whelped if mated. No treatment is usually necessary, unless the bitch actually produces milk, in which case you can get something to dry it up, or else she may get mastitis as there are no pups to suckle and reduce the milk.
Main thing is more exercise to take her mind off her 'babies' which you should not remove or else she may fret. Just let her carry them round for a few days and you will soon find she is back to normal. If you are into homeopathic treatments, pulsillata can help and if she gets milk then fennel will help dry it up. Spaying should stop it, but not always, and you can also have an injection called delvosterone, which is the same used to prevent them coming into season. Its not a real medical emergency, they just make you feel very sorry for them with their big sad eyes as they carry their 'babies' around.