The same sort of thing happened to me, except I was measuring big. I wasn't concerned because my first 2 babies were ~9 pounds. But I wasn't going to turn down another ultrasound, ya know? BTW, I was seeing a different person that day because my doctor was in surgery.
During the u/s, the baby wasn't measuring big at all. In fact, he was only 75 percentile. I thought that odd since my other babies measured bigger.
Then just a few weeks later, I was measuring a week or two small. My doctor wasn't concerned, although I was a bit confused.
Turned out my baby was only 7 lbs 7 oz! He was tall (22 inches) and skinny and not at all what we expected. But he's perfectly healthy.
When I was trying to figure out what was going on with the measurements, I realized that when they measure us, they're using centimeters to measure something that's soft and round and internal. Talk about a fudge factor! Take another look at a ruler and see how tiny a centimeter is! I think it's a bit ridiculous, when you think about it, that they order tests when there's only 2 cm difference between what they measured and what they think they should have measured. As if they could possibly measure that accurately!
So I'm sure your baby's fine. Enjoy the extra ultrasound!