I would have thought it was too early to spay her, normal advice is to let the milk dry up, which is normally a week after they have finished feeding the kittens (or after they have been rehomed), which also gives them chance to put a bit of weight back on after feeding, if they are underweight, the op is more risky. If she is in heat, it is more risky to neuter, and some vets wont do it.