Head measurements

Sooo I am so stressed out after my appt today. I am 31wks pregnant with baby #3 ... I have been going to routine ultrasounds due to the babies weight being lower than normal. Well today I went for my ultrasound and I was told the baby is now measuring normal at 3.14 pounds but her head is measuring smaller then her body ?! I was told at all the other appts that everything else was measuring fine it was just her weight now I’m slammed with this !! I have never had thes issues with my other two pregnancies. Any one else experience this ?! Please help