I wouldn't have spayed her while she is nursing. You're very lucky she didn't die during the operation - there is an increased blood flow in the area considering she's producing milk and an increased risk of blood loss during the operation. Nursing will be very painful for her considering she was just cut open next to her nipples. For a spay in good conditions you should give meds to avoid infections (antibiotics) and to avoid pain (Metacam / meloxicam) for at least 5 days after the operation but if the doe is nursing you can't because it will go into the milk. The only case where the spay of a nursing doe is recommended is when there is a chance she is pregnant again (when the male was there while she gave birth). In your case, it was unecessary.
It's too late now, but you will have to monitor your poor girl closely - considering she can't be given pain meds there is a chance she will stop eating because of the pain (the combined stress of a first litter with the stress of the operation doesn't help) which would lead to GI stasis. Make sure she eats plenty of hay.
Also, find another vet. He could have killed your rabbit and I wouldn't trust a vet doing 'promotions' on operations anyway.
PS : don't forget to get the babies sexed by a knowledgeable vet when they are around 8 weeks old and to separate them as soon as it's done if you don't want any incestuous teenage surprise litters!