I read this book in the early 1990s, but have no idea when it was published. The details I remember are that there is a girl who is for some reason befriended by fairies. She shrinks down and goes to visit them. Maybe she’s going to become their queen or something? For some reason she ends up with this necklace made of thorns that she can’t take off. The only other thing I remember is that the fairies would feed her nectar from a flower.