Worried

My dr called and said they have to redo my ultra sound. We already know the sex for sure but they said they couldn't see the face measurements and she measured a week small according to what little measurements they were able to get. They were worried. She was basically jumping on the spot the ultra sound tech was poking at. I know she's active and ALIVE  which is the most important thing but I'm worried that there's something else wrong. Because why else would they want to do another ultrasound so soon with such urgency?