The BMI is a flawed guide, with "guide" being the operative word. I'm *JUST* in the "normal BMI" range now, and I'm a US6-8/AU10-12, at 5 foot 7 inches. My collarbone, neck tendons, chest bones all stick out, I can see my ribs, my muscles and tendons in my arms etc. I can even see my hip bones through the loose skin. I look skinny - I know I do, because I get a shock every time I see my full-body reflection, and I've been called skinny by others. But my BMI and the Hip/Waist ratio markers all suggest I'm not. My Hip/Waist ratio tells me I'm still quite overweight! It really doesn't take body shape into account as I've never had an hourglass figure - I joke that I'm shaped like a plank with tits. Straight up and down.
So don't take the BMI to heart, and I think it would be a good idea to tell this doctor your history. Some doctors don't think beyond their charts, unfortunately.