Baby measuring small at 22 weeks

This is my first pregnancy and everything’s going great. Had our 20 week anatomy scan and everything was on track, then at 22 weeks had another scan with perinatologist (they’re following me bevause I’ve had a blood clot), and they said again everything looked really good, but she is measuring “a little small.” They said she was 15 oz which seemed normal to me. They want to see us in 4 weeks for another ultrasound to check growth. Now I’m worried and I’m sure it’s fine because all babies grow so differently, but has anyone had any experience with this??