This is a purely hypothetical question. This is not a predicament I would ever place someone in.
You used to be a loyal servant of Mistress "A". She has told you something in confidence and you assured her discretion. For whatever reason, that relationship is no longer and you now loyally serve Mistress "B". She asks you to reveal this specific thing. What do you do? Do you remain loyal to your previous or current Mistress? Would it depend on something? what?