i had a very similar problem with bella. i have an all white bed and she would seriously pee on it about once a day when i first got her.....i was about to lose my mind i was washing my sheets and down conforter cover soo much.....the only thing that helped me was to prevent her from coming on the bed, unless she had just gone out and i was watching her.....now she is fine....but for a while there things were not so good...hehe
i hope that you figure it out...good luck!!