I probably wouldn;t have said much as I avoid confrontation like the plague - but would have felt plenty.
Was she a nurse affiliated with your surgeon's office, or does she work in the bariatric ward at the hospital? I'm hoping that maybe she was a general ward nurse and just really doesn't get it. Patients lose weight on the pre-op diet because they have a clear goal in sight, and know that it is very short term. It is rather idiotic to be surprised at someone who has lost weight on an extremely low calorie diet. Duh. But it could not be sustained in any viable or healthy way.
I was considered low BMI for this surgery too - I just about squeaked through qualification with my co-morbidities. Yes, I did get people who were surprised at what I was doing and told me that I didn't need it. Oh, but I really did. 20 years of health problems that started as low-key but became harder to ignore with every passing year dictated that I do this now, or do it later when I am even heavier and more at risk. It really isn't up to a nurse to be gossipy and nosy about your health situation and the private battles you have fought. Hugs to you.