I bought size two pants yesterday. Utter mindfuck.
Three years after my diagnosis, I still do not see myself as thin. It's jarring when anyone points it out - when my mother says my arms are "so thin;" when we run into someone we haven't seen in a few years and that what happened to you question flashes on her face. Part of me, rationally, knows how I look. When I see my reflection in the morning, I acknowledge that I am thin. But outside the privacy of the bathroom, there are no guarantees.
People don't understand this, and I don't blame them. Not everyone is especially fond of their bodies, but most people have a good idea of what they look like. They can predict how people will react to their appearance, they know what size to look for on the rack. I can't do either - because I don't have a stable body image. From day to day, from meal to meal, the way I see myself changes. It's profoundly disorienting.