Haha, I lost a lot of weight (80lbs) right before getting pregnant. So, people I hadn't seen in a while were complimenting me on my weight loss until I was like 32 weeks along, even though I always had a D bump. Then when I had about 7-8 weeks left to go, suddenly everyone started saying "I didn't know you were pregnant!"
People kept telling me I was carrying small, but I think that's because there was another girl in the department next to mine who was 9 weeks ahead of me who became a freaking HOUSE while pregnant. Her bump was like twice the size of mine. Yet my baby weighed more at birth.
Another thing that concealed it for me, I think was that I carry my weight in my hips and thighs so my waist is quite small by comparison. So, having a big belly just kind of made me look like a regular fat girl.