Hi claire, isn't Cassie a worrier, if I've got the right dog, did she used to wee when you had visitors, if it is her, I would say its just anmother submissive behaviour because of her lack of confidence. When dogs are worried they can use all manner of submissive behaviours from rolling on their back and showing their bellies, slightly lifting the back leg, showing their top teeth, weeing, excessive licking and jumping up. Keep working on her confidence building and she will stop the behaviour on her own.