Terri-poo pees on bed

Hi
I have a 7 pound little terri poo who has decided she like the softness of my bed to pee on. She just turned 2. Missie has has a tough year,she lost her buddy Dallas a 90 pound huskey to cancer in January. She went through major depression and lost 20% of her weight. I got her a cat ( Piper ) to help but she needed a doggie friend. So I am now the foster parent to a Shih Tzu.
They got along great until Sophie had to go back for breeding. After a week Sophie came back and now Missie will pee on my bed. Any clues to why she would be doing this? Jealous furbaby syndrome? Missie is healthy and she is just starting to play with Sophie again. The vet checked her out and couldn't find anything wrong with her.
Thanks
Shaykeija

Before anyone thinks Sophie came or is from a back yard breeder I can assure you she is a registerd with the Canadian Kennel Club. Her sister just won an internation award in New york for best of breed. I detest byb's. Piper and Missie were adopted from the pound. Still any help with Missie's pee problem would be appreciated. :)

Is Missie spayed? If she is, and the other dog is not, this could cause her urination. Intact animals sometimes make spayed/neutered ones nervous and they pee to spread their scent around and comfort themselves.

Does the other dog get on that bed too?

Neither Missie or Sophie are spayed. Missie will be going in at the same time Sophie does for spaying.
Thanks for the help. :D