37w4d pregnant, please read?

I went in for a ob growth ultrasound today because my belly hasn't been measuring what it should be for being almost 38 weeks. From this ultrasound, they concluded my baby is only a little over 5 pounds. I have been gaining the weight but she is not retaining it, can someone please tell me why this may be? I am an emotional wreck. They put me on bed rest and all I can do is worry. Next week I have 2 appointments, one of a non stress test and the other for a followup ultrasound. Can someone help me understand what is going on?