Just goggled this, there are quite a few sites about waiting 3 mths, but thought you might find this interesting.
Neutering Your Dog | Castration | Spaying | Puppy | Holly House Veterinary Hospital
"ideally we will wait 3 months following a season to spay a dog as there are increased surgical and hormonal risks if spayed too soon after a season."
"Transient false pregnancy can occur in some bitches shortly after spaying, especially in older bitches. This is more likely if spayed within 3 months of a season. It is easily treated."