There's your answer.  Whenever she says anything to you about your food choices...cock your eyes with a slightly confused, slightly insulted look on your face and then tell her that you are happily engaged. Then turn around and walk off. Don't give her a chance to respond. Just get out of there. Leave. And then casually mention to your supervisor that this woman makes you "uncomfortable". That you're engaged and you think she's been flirting with you. She'll never speak to you again.