My kitten is sleepy and lethergic?

I have an 8 week old male & female kittens (brother & sister). We collected them yesterday morning from a cat rescue centre. They were both vaccinated (first treatment) on Friday. The male is lively, eating and going to the toilet fine. The female is lethargic, with runny stools and has vomited a couple of times. She is drinking water OK, but has not eaten. Her breathing and heart rate appear OK. Because she would not eat we have syringe fed her cat milk. This morning (4 hours ago) we took her to the vets as we were so concerned. The vet said her heart and temperature was fine. He gave her a broad spectrum anti-biotic injection and an injection to stop the vomiting. We have been given some paste that should thicken her stools and contains pro-biotics. We are now also syringe feeding her a pate/water mixture. Despite this her condition has not changed. She is still sleepy and lathargic. I am probably worrying too much, and the fact we lost our first cat to an abdominal tumour 2 months ago is fresh in my mind. When should we see any change in her?