Baby's belly measuring in 5th percentile?

well i just got back from my weekly appointment and the doctor made me get an ultasound because i aam still measuring pretty small.. well the baby is about 6 lbs 12 oz which is in the 15th percentile and her stomach is measuring in the 5th percentile.. i am soo worried i have been a mess since i left the doctors.. am waiting for a call back to see what the doctor wants to do :/ verrrrrrry worried.. anyone else have this happen? also i am 39 + 4 weeks