When I had my ultrasound at 11 weeks (last pregnancy) we were watching the screen and Aston was jumping around in my uterus - yet i couldn't feel a thing. but at about 13-14 weeks I started feeling 'fluttery' feelings which I think were him.

Probably also depends how thin you are and how much gas you're otherwise producing - cos Pregnancy does produce lots of gas which burbles around your insides for ages.