I'm sorry to hear about this. When I was young, I was underweight. Complete strangers would walk up to me and my parents and say, "My gosh, she's SO skinny." But that was just me; I ate a ton and was bony.
That unhealthy body image stayed with me until I was in my 30s and filled out enough to not look bony any more. Finally, one day, I looked in the mirror and realized I wasn't skeletal anymore.
I'm very petite and don't need to eat much. (Unfortuantely, I eat more than I need, thus my current weight issue.) But lots of my coworkers would say, "Is THAT all you are eating for lunch?" It was all I needed. I'm not sure why they felt they could just comment on my food intake.
It's a conundrum.