how long did it take for your stomach to get hard?

ok well i’m here again just freaking out as usual cause i might be pregnant(12 weeks) and but i’m not for sure. well for the past two weeks now my stomach has been feeling hard but i took 3 tests(all negative) but i still feel like i’m pregnant, i’ve been getting really bad back pains if i sit down/ stand up for too long, my stomach itches, i’m always hungry and i visit the bathroom more often.but i’m getting an app to get an ultrasound but i just wanted to know from people who experienced it before. thanks