In these circumstances, everything I have found says you can spay a cat even if she is still nursing, and she will still produce milk for her kittens. It is a slightly more difficult surgery, so check with your vet if their office would be okay with that. Usually it is recommended to wait until kittens are weaned, but as a cat can get pregnant again a week after giving north, and you have no control over her, really, they may make an exception.