At 12 weeks she is still a baby - so the best thing you can do is shut the door to your bedroom until she has matured and you can really trust her. It is possible that the first time she was asleep and woke up disoriantated and used your bed. Have you washed in bio washing powder or used Simple solution - otherwise you will leave trace smell for her (that you can't smell) that will tell her it is a good place to good. Give your mattress a wash too.

Hope that helps.

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