That would be a massive (over a third - just think of that with yourself) and frightening loss. It would not be explained by surgery. I am disinclined to believe the loss, particularly with her appetite, but do tell the vet about it. It is possible the 3 lb. vet record weight was another kitten.
Jan,
I'll ask the breeder if she took more than one in, but I was under the impression Madisyn went in alone and the rest were done on Thursday. She wanted Maddy to fly today so she was the first spayed. She did say she had dropped some due to the surgery, but she didn't say how much but said she was 2 pounds. I guess now you know why I was concerned. There must be an error somewhere. As long as she is as you say alert, active, playful, etc......I'll try not to stress anymore about it but will be sure to point the discrepancy out to the vet.
The breeder may know the 3 lb record is wrong, no matter the source of the error. (Keep in mind the parts of pounds are important at these weights. There is a big difference between a drop from 3.1 pounds to 2 pounds and to 2.9 pounds.)