That is one thing that I have learned from therapy. We are all guilty of choosing for others how they are going to react, what they are going to say, before we even talk to them.
It is worth giving her a chance, maybe she will not be nasty.
Also, maybe try to talk to her from a different angle. Think about how your relationship works at the moment, then tink and try about a different approach.
Whatever the outcome, it is worth sharing your feelings and worries with your nana. Good luck xx