Possible cat in morning

I don't know how many read my thread about my 2 lb Yorkie, Cujo. He had a rare disease and we lost him about a week after he was finally diagnosed, in June 07. Well, about three weeks ago, we had his mother neutered and she did not wake up from surgery. This was not only upsetting to myself and my husband, but we think my son's cat is extremely lonely. The reason I say this is because, both of the dogs lived upstairs with the cat and played together all day long. Now, the cat is by herself. Even though I try to give her more attention, I discovered that while I was at work, she went into the spare bedroom, where her litter box is and urinated on the bed. I tried to clean it as much as possible, but she keeps going back in different spots to urinate. This cat has lived with us for the last 6 years and has never had any problems with urinating anywhere except her box. All I can figure is that she is upset and for whatever reason she is using this method to get my attention. I have covered up the bed with plastic bags to discourage her from jumping on the bed, until I can figure how to dispose of the mattress. I do not want to buy another bed until I solve this problem. She does still use her box, so I do not think it is a bladder infection. There are no other symptoms. Any ideas?