Well, I think the others are right with it being a name you call her, but not derived from
Charlotte.
I have something similar, I want to name my first daughter
Catriona Ena Morgaine and call her Minnchen (from
Minna). That will me my "personal" nn for her, it might not be derived from her real name, but it has a meaning to me and so I don't see why not.
I know so many people calling their children "pumkin", "tiger", ...

