Thanks for your reply. The kitten hadn't gotten into anything she shouldn't have and we hadn't changed her diet - I couldn't think of any possible reason - which was so worrying. Anyway, we made it through the night and the vet diagnosed inflamed intestines - apparently this sort of infection is not unusual in kittens. She was given an injection, medication and dry kitten food and we'll go back to the vet on Saturday. She has already got a little better - so fingers crossed!