As long as you can feel movement and your doctor is happy with the fundal measurement I wouldn't be too concerned. Even up until a couple of weeks ago I had people saying how compact my belly was even though my OB says I'm exactly on target and all my ultrasounds have shown growth to be on target, one scan showed me at being 5 days ahead of LMP EDD. I've even had people not realise I was pregnant during my third trimester so obviously I didn't look all that pregnant.

I've only felt the heaviness of bubs in the last few weeks, even now when I'm lying down I sometimes feel so normal I find it hard to believe there's a big belly popping out down there. Then I go to stand up and I sure feel it.