Dog Keeps Peeing on the Bed

[Angela]

I have two shizu dogs, Mookie who is almost 4 and Minxie who is 1 1/2. In the last 6 months Minxie has started to pee on my bed. She will do this on any bed she encounters. I have no idea why and she has never done this before but no matter what I do to discipline her or to prevent it, anytime she is alone and gets a chance, she will pee on any bed. Once she even peed on it when I was sleeping on it. She has previously been very good and let me know when she wants to go outside or has peed on her pee mat when if it was in the middle of the night ( I live in a condo so I can't let them out in the middle of the night).
Any ideas why she might be doing this?

Thnx!