
All bitches go through phantom pregnancies to varying degrees after every season. Some bitches simply become a little quieter than normal, others go the whole hog producing quantities of milk and guarding imaginary babies. These ones can make themselves quite ill, and the only cure is spaying.
I'm a little concerned that the vet intends to operate so soon, because usually they won't spay until there are no symptoms of phantom pregnancy (lactation etc) to avoid possible behavioural problems later. The usual time to spay is about 3 months after the start of the season, when hormone activity is at its lowest level.