I don't remember much else, I think she got into the fantasy world in a vaguely Narnia esque way, ie fell into the world without knowing it existed.
She ate some food which a jailer creature gave her, it was icecream etc, then it transpired the food wasn't real and she had a hollow feeling in her belly. She also had a hot bubble bath but it was similarly an illusion and she realised she was still dirty after.
Is this ringing bells for anyone?
Published around 1990, though possibly first published earlier