some of my kittens like to go on the pile of washing waiting for the machine , even though there are plenty of littertrays around the house and another favourite place is behind the sofa ..............if its when you are not in the bed you could try putting a plastic table cloth over the bed and maybe even putting her food on a tray there . she may not like where the litter tray is or the litter you are using or else she is just trying to be controlling