Extreme weight gain

At my 23week appointment my dr told me that I had only gained 5 pounds, which was ok but slightly low. At 26 weeks they weighed me and I had gained 4 pounds for a total of 9 lbs overall. I went in again 6 days later at 27 weeks and I had gained nearly 4 more pounds in 6 days!!! I haven't changed anything in my daily routine, no added stress, same amount of movement and exercise, same food and same intake amount. Gestational diabetes test came back negative.  Does anyone have any idea why I'm suddenly packing on pounds?!