I have two friends who had a similar situation. One was slightly overweight pre-pregnancy, had horrible morning sickness, and ended up weight 10 lbs less than her pre-pregnancy weight at the time of her delivery.
Another friend of mine was really tall, and didn't show until the 8th month and even then looked only about how I looked at 5 months pregnant.
Every one carries/gains totally different. If everything looks good with baby, I wouldn't stress.