Not sure why she has started doing this. So only have 2 ideas, hae you tried getting up to let her out in the night or have you ever used a crate for them, could you crate them overnight as dogs don't like to toilet in their beds.
Good luck - hope you get it sorted soon.

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A dog Possesses Beauty without Vanity,Strength without Insolence, Courage without Ferocity, And all the Virtues of Man without his Vices.