"The Book of the Bitch", a great breeder's reference IMO, states, "Some breeders argue against this injection (oxytocin) being given, as it can seem that a bitch which has been passively content with its puppies becomes excited and stressed again under the influence of the hormone, but this stage soon passes."
This was a 1st time mom. She had 5 pups over about a 5-hour period and the breeder thought she was done. 5 hours later she had another. The next day she was running a fever of 104 and the owner was concerned that there could have been a retained placenta. The vet gave a shot of oxytocin and a round of antibiotics. She recovered quickly.
Without a doubt, the stress of a trip to the vet so quickly after whelping could have been enough to upset the new mom, but oxytocin is a powerful hormone.