Measuring small

Kaitlin
So just got done with my 36 week appointment and the doctor said that I'm measuring a little small so he is making us get another ultrasound next week. I am a FTM and have gained about 20 lbs. I don't understand how I can be small as I feel huge. I'm seriously so terrified that something is wrong with my little girl! Has anyone else experienced this or have advice?!