Before anyone judges me....I am the one that rescued the dog I am questioning about....?

A person found a stray female dog with 4 puppies that looked to be around 2 weeks old. I found out after the lady had had her and the puppies for about a month. I got mom when the pups were 9 weeks old and already in homes. I made an appointment to have her spayed 10 days later. When I took her in I was told she was already pregnant again! I was shocked! She had not been around an intact male during the 10 days waiting to be fixed. From what the vet said they were more then 10 days into gestation. I know this may sound cruel to some but the vet did the spaying. My question is: Never being around a pregnant dog with newborns, can a dog put themselves in heat like a cat can? Please do not be judgmental.