That sounds scary Ella! I'm really sorry you are experiencing this.
It sounds a bit like a form of dissociation -
Mind website on dissociation. Or it could be psychosis -
Mind website on psychosis.
I guess the reasons I mentioned the above is because there are different way s of approaching your experiences depending on which of the above it is... If it's dissociation then you could try talking to her to find out more about her, why she's following you, what she wants etc.. If it's psychosis then I've been told it's best to ignore it, neither trying to prove or disprove it, because thinking about it causes your mind to focus on it and reinforces the delusion.
Hope this helps a little

Thinking of you hon xx