Fundal height measurement

Just had my 29 week appointment and the doctor said my weight gain, detailed anatomy scan from week 20, ironed samples, and the baby's heartbeat all looked great. Then she measured my fundal height and said I only measured 24 weeks. So she gave me a script to go in for another ultrasound to check on the baby's development. My doctor didn't seem super upset because I was a small baby and my mother measured small in her pregnancy too.