Ask her outright why, in her opinion, she thinks you should diet or fast. Also ask her if she makes a habit of asking personal and impertinent questions, or is there some reason why she has singled you out. Then tell her, unless you actively intend to seek her advice on diet as clearly she is some self appointed expert on the matter, perhaps she'd like to keep her misplaced thoughts and opinionsto herself. Warn your husband beforehand that because she makes you feel uncomfortable you are likely to retaliate in kind.
Good luck.