Small fetus

Hi, I am a little worried. Saw a GP yesterday for the big scan and he told me that my baby was small; 619gr, for 24 weeks and 4 days. I haven’t put on any weight in almost 2 weeks either. But I got a toxoplasmosis and a Down syndrome test done and they were both negative. It could just be a small baby but I am still worried. I’ve had quite a bit of stress lately but I’ve been working out and I have a healthy diet. Has anyone had similar experiences?