Hmmmm as a parent, who had a rat (yet alone mouse) bought into my house on more than one occasion by my cat ... it's not a pretty sight.

Out of context, I would be mortified if someone heard that my house had a rat - without knowing the story in context.

I would genuinely have had the same reaction as your mother initially did if one of my kids told me they had told xyzzy we had a rat in our place.

Obviously your therapist gets it, but I guess your mother just needed some assurance that she got it.

Unfortunately they do carry all sorts of who knows what and have quite a stigma. Don't take her reaction to heart. I think you're reading too much into it