Could be two issues. First would be stress/discontent. If this is the case, I would put a fresh litter box in the bedroom temporarily in the hope she will go there. If you do not already have at least two litter boxes, its a very good idea with multiple cats. The second issue could be medical in which case she may not be purposely doing, but rather she is lying up on your bed and then suddenly can't help herself. I would first try the litterbox for a day and show and spend some time with her. If it does not get better quickly, definitely visit the vet