I am the same as you, I was showing so so early on and you should see me now, I'm so tired of everyone telling me how huge i am!
I think too if you've been pregnant before you pop out more quickly.
My friend is 2 weeks further along than me and she isn't showing at all where as i'm bigger than another friend who is 28 weeks.
I keep thinking maybe i've got another baby hiding in there somewhere haha.
Good luck Twins would be great hey? either way hope all is well and you have a healthy and happy pregnancy