31 weeks appt . Belly smaller ?

Channing. • baby # 2 . My husband and I have been married 6 years and our 4 year old can`t wait to have a sibling .
I have been measuring 1 week ahead since 9 weeks but at my last appt my belly had not grown from 29-31 it measured exactly the same . Dr did not seem worried but of course I am . Anyone else have this ? Is it normal ?