I started as solid A (likely due to meds in my young adult years), I passed a B at about month 7-8th and i am now a C as of fairly recently. My rapidity and results are not typical but i am thankful, although it caught me by surprise.
My other body changes have been fast too - my butt is larger, my waist is smaller and my skin has changed a lot. My doc says it happens that way sometimes. Net weight gain has only been about 3 pounds, but i wish i could reduce. The only thing i have not seen is facial changes that i can detect at least.
So yes the changes can come very fast. I believe i have new stretch marks as a result.