That's amazing! Have you used the technique, OS?If she does come into call you can stop it with accupressure mating - look towards the bottom of this article:
http://www.messybeast.com/catmassage.htm
I do second this answer, as far as I know.Depends on risk of her getting pregnant again. I had my mom cat done when her babies were 8 weeks. I also have had a mom done who was only few days after birth(there was fear of retained kitten) She actually did fine and nursed her kittens for months! Both had no chance of getting pregnant(they came to me pregnant/with babies and I had them in room away from to much activity.
Just make sure the vet clears it and knows as they may do the spay a bit different [in the flank perhaps?] to keep risks low of the kittens tearing anything and to make it easier for her to continue to nurse.