What is the matter with a cat I'm looking after?

you say you were given the cats to look after which suggests they are staying with you. Like humans new environments can be exciting at first but after a while they may start to feel a bit homesick. She's probably just missing her home and her owners. Keep an eye on her and give her as much love and fuss as she wants. As long as there's no other symptoms and she's eating ok it's unlikely to be too serious.