Can someone explain this?!

My brother's cat went to get fixed yesterday (finally), and something weird happened. Now, at the beginning of February she had kittens. (Not my fault; if it'd been up to me she'd have been fixed last November), but she had completely weaned the kittens before we took her to get fixed. However--after the surgery that was supposed to fix her (it was performed by the SPCA clinic, so I'm pretty sure they did)--she's suddenly producing milk again, and in the same quantity that she did when the kittens were first born. What the heck?

I had the same question
Here the thread
http://www.thecatsite.com/t/295051/why-producing-milk

If the kittens nurse, a recently spayed mother can produce milk for them.  One of my sweetest rescue stories is about that, where a recently spayed cat took on an orphan litter of kittens, produced milk to last their whole kittenhood, and actually fostered several more litters before kitten season was over!

In your case I don't know if her existing kittens are still with her, but kittens do nurse for comfort and I've seen a litter of 6-month-old kittens nursin right along with mama's second litter who are maybe 5 weeks old themselves.  I have photos.  Anyhow, the vets did nothing wrong, your cat has been spayed, and for some reason she is producing milk.  When no kittens have nursed in about two weeks, she will dry up.