Another cat update...
Not looking good for the smallest of the two cats I'm afraid. She has always been a bit sickly (as documented here) and she hasn't coped well with being spayed today.
Right now she's back at the vets for the night because she became weak and couldn't stand up, lots of dribbling too. She's seen a different vet tonight and they think she may have a Portosystemic Shunt
in her Liver, it's a kind of "short circuit" that stops the blood being cleansed by the liver and the nutrients being absorbed properly. It would account for her odd behavior and lack of growth, blood tests tonight may confirm this.
I think it's likely that she will be put to sleep, she's very ill tonight and even if she recovers from this bout she'll probably be left with a serious condition which isn't going to get better.