Maybe she's afraid that you all will be eating tempting foods in the lunchroom or have snacks at your desk and that before long she'll be joining you all in fatland again. I don't know how she lost her weight but I know some people who have been through OA (a 12 step program) and they were encouraged to stay away from old eating buddies. Maybe she's afraid that other large people will become new eating buddies. Or maybe she can't control herSELF and is afraid that she'll start preaching about weight loss and alienate people.

Then again, it may be none of the above. How does she act around the thin people in the office?