Dear M. S,
The exact same thing happened to me 22 years ago! I was so scared. Back then they didn't do routine ultrasounds and my Dr. wanted me to have one because I had not gotten any bigger (at least not in the measurement around my belly- I had gained 50 lbs! ;-)
This is how they explained it to me. They measured the biparietal diameter, a measurement around the baby's head and then checked the proportions of the baby's body. They told me that sometimes when the growth is not progressing the body sacrifices its growth for the brain development. All of this is normal. They told me, based on the ultrasound, that she was either small or I was wrong on my due date (which I knew I wasn't). Two weeks later I gave birth to an amazing 5lb 7 oz tall, gorgeous, bald baby girl. She is now 22, fantastic, health, bright, with lots of blond hair (3 inches taller than me). She is in her senior year at UT Austin.
I agree with the other mom who posted and told you not to worry...you know your baby is fine.
P.S. Ten years later I gave birth to a 7lb 9 oz boy!
Congratulations and Best Wishes!
D.