I totally get what you're saying. My mom (whom I love dearly) drives me up the wall. The fact is that she means well, but I can hardly stand it when she tells me, "why do you need to eat that now, you just had something!" For goodness sake, I'm in maintenance and she's STILL on my case. Sometimes, when I was heavier, she made comments about how I couldn't control myself in front of other people! She didn't even realize she was doing it until I pointed it out to her and then she was quite horrified. She thinks I"m overly sensitive.
My best advice to you is to tell her that from now on, she's not to comment on anything you're eating or anything weight related. That's between you and your food. But I can definitely relate!