PREGNANCY BELLY?

If your pregnant does your stomach go hard? Like if I was to be say 17 weeks would my stomach feel hard to push on? I'm quite skinny (size 8 AUS) and my stomach is giggily. It only feels like pressure when I push on it. 
I have taken 12 pregnancy tests and they have all been negative and I have got my period 4 times since I last had sex. I have been to the doctors and they wernt concern about pregnancy. 
Could I be pregnant?