Measuring behind

So I’m at 34 weeks and we went to the doctor yesterday for our growth scan and now they are concerned that she is tooo small. They said she weighs 4 lbs 4 oz but all of her measurements are showing she is in the 14th percentile and that’s borderline something to be concerned about. We go back on May 1st for another ultrasound and if she has dropped percents at all they are going to deliver. I’m a little nervous because I thought she was right on size but they didn’t tell me exactly why they were concerned..