Yup - I was another who was average-sized and fit and was (forcibly) convinced I was fat! Starting around age 12 or so, my mom's mantra was how "lucky" I was that SHE didn't tell me I was fat, when I weighed 20 pounds more than SHE did at my age and her family teased her about it. Yeah... somehow, all that came through was that a) I was fat, and b) I was SO fat that my mother needed to comment even though she didn't want to. (Nevermind the fact that I had 4 inches on her and a totally different build!)
I actually came across an old picture of myself at about age 14 this weekend. I know I probably weighed about 150 or so (at 5'6"), (and I only know this because my driver's license weight STILL says 140, which was what I was LYING about at age 14!). I thought I was hideous and fat and all sorts of things - I'm holding my arm in front of my "belly" and was uncomfortable in the sleeveless shirt:
I literally cried when I was looking at this picture - how I wish I could tell that poor girl what I know now!
The funniest part is that I have the opposite experience now - I think I'm thinner than I am until I see pictures! Back then, I thought I was a cow and the pictures show otherwise - now I think I'm pretty darn hot (and, frankly, I am!) but occasionally a picture shows I'm bigger than I realize.