And if you really want it to end, be firm but pleasant. I have had similar things happen to me, too, and others can push with: 'why?', 'why not?', 'is there a problem? etc, etc.
Don't be dragged into a long conversation over it.
You can give an explanation or not, but, try to keep it brief and without opportunity for her to change your mind against your will.
Good luck!
