Worried

So I had a Doctors Appt. today for a fetal growth ultrasound . And the doctor told me my baby weighs exactly 2lbs and 5oz and that she has a low birth weight and they want to monitor me twice a week to see if she's developing ok . But when I read online or on apps ( just like this one ) it tells me that what my baby should weigh at 28 weeks . I'm really confused 🤔😰