Not diabetic but not breaking down sugar?

I went to my Drs appt today and an 31 weeks and 6 days....and the baby is weighing 5 pounds already! Which put the doctor into a small panic. I was overweight when conceived and have  gained 20 pounds so far. I watch what I eat and exercise. My doctor told me that the baby is growing too fast obviously and that my body is not breaking down sugars/insulin. However I have done the diabetes test both the one hour and three hour test and I am not diabetic. Now I have to monitor my sugar levels and so far all have been good I have not had high sugar. Is this happening to anyone else? And if I'm not diabetic why is this happening?