It's not common, but if you know you're eating healthy, and you're not underweight, and your baby is showing all the signs of being healthy, I wouldn't worry too much about it. Truth is, there is a wider range of "normal" than what a lot of doctors will allow.
Every woman grows their baby differently. I knew a lady who lost weight her whole pregnancy, and one woman who only gained 5 pounds. It just depends on your body. As long and your baby is healthy and growing and you know you're taking care of yourself then there's not much to worry about. Ask your OB if it's bothering you too much.