Babies size??

Okay so I've been measuring about 5 weeks ahead by stomach and 3 weeks ahead by ultrasound. I failed my 1 hour glucose test then went back in for my 3 hours glucose test a few days later. When I called they said I had passed. Well I went in today for another appointment and the Dr I went to today said she doesn't know how I passed it because I was literally a hair away from failing it. She also said that is probably why my baby was weighing 4 lbs at 30 weeks. I'm just confused how could I have passed but it still be the reason my baby is growing so big so fast? Wouldn't they want to monitor as if I did have gestational diabetes? I just don't get it.. If she gets to big and they aren't monitoring it properly as if I did have it she could get huge.