How do you mean by swatting at her?

Do you mean using her paw? If so, she could be trying to pull the pup towards her and our Mist did that when we were handling her pups for any reason. She'd use her paw to try and get them back. They can be a bit clumsy in how they go about it but thats dogs for you, lol.

Mist would also want to mother the pups of my other dog, Jess. In fact, with Jess's first litter, Mist actually started to produce milk for them and would suckle them. She went mad when she heard them crying and was desperate to be with them. Fortunately, Jess was a little inexperienced that first time and she was glad for Mists help.

Your bitch's maternal instincts may have been triggered by hearing the puppy cry etc...I'm sure she could still feel maternal towards puppies even though she has been spayed.
It's worth a try to see if she'll act as a surrogate mother.

When the puppy is a bit older and started weaning, going back with it's siblings might be a good idea. I hand reared two pups of our Mists from a large litter and they went back with the others after about 3 and a half weeks and did just fine from there on.

See if you can get your bitch to lay down in her bed or whatever and then put the pup in with her and see how she reacts. Stay close so you can pick the pup straight up if things don't go well but, in many cases, bitches do take to small puppies quite well and develop a desire to protect them.