Is there a more enraging question? I've experienced this a few times in recent months. Once from my partner [ who was being light-hearted but it didn't land], and twice from my kids in recent months.
Now, I am always careful not to announce that "I'm on a diet" - I dont believe in them, as such. However, I do try to limit my excessive greed at certain times of the month and up my activity levels when the jeans get tight. So, the kids probably do see me eating better at certain times, and not so good at other times. But, at the weekend I decided to do some baking because I have an excess of eggs delivered. This led to my DD asking "but I thought you were on a diet?" 
Its so rude and judgey, and I told her so. One time I would ignore, but the fact that a couple of different people have thought it appropriate to comment similar to me in recent months has made me paranoid and [still] annoyed...