Yeh, when flaring I can drop lots of weight off. One flare I had that landed me in hospital, years ago, saw me lose 6kg in less than a month (Im not that big, so this was drastic). The hospital bed I was put into was in the adolescent anorexic ward. I remember the young girls thinking that I was an old anorexic, then explained why I was there and how I wish I could tolerate food, gave them food for thought, although one of them wished she had the disease herself!
When I get too thin, I supplement my diet with smoothies and lots of snacking in between meals.
Hope you find something to help regain weight, its not a good thing to be too thin.