To be honest, she's being a nut. Yes, her heart might have started in the right place, but badgering someone EIGHT times in ANY situation is way too much.
After about the 3rd time, I would say something like, "I really appreciate you thinking about me, but I can not have any. Please stop asking."
And any time after that, I would simply say, "You have already asked me x number of times. Stop asking."
You do not have to educate, justify, or explain to her, as it is really none of her business, and this is not a situation where you asked her to buy these items for you. This is a workplace, and she is spending an awful lot of time harassing a coworker about food.