15 weeks and don't feel bump??

Hello, 
I'm 15 weeks and I don't feel a bump, is that normal?? When should you be able to touch your stomach and FEEL something hard? 
I was laying down right now trying to feel for my uterus but there's nothing, I should also mention that I am overweight but I lifted my belly out of the way (gross I know), and felt under neath it, on top, close to my pelvis and there's nothing. 
I have my gender ultrasound later today and I'm just worried that nothing will be there. 
Am I freaking out for nothing or should I feel something when laying down?