Don't worry about it Rusty - everyone is different. Usually you will notice your belly 'pops' when your uterus gets too big for your pelvis. It sits inside your pelvis usually and as it expands, you won't really notice it until it starts to rise out of the rim of your pelvis. Does that make sense?
I know with DD, one day I was wearing all my old clothes and had a lovely flat tum and literally, the next day, I was out buying maternity clothes. But then my belly stayed the same size for months before it started growing again.