so confused…..could i be preggo

hey well let me just start by saying i took 3 HPT and all negative, 1 too early and the other 2 about a month ago. well for about a month now my stomach has been feeling hard and i’ve been getting back pains(that i never had before) when i sit down/stand up for too long. well i had 2 periods since oct, 1 3 days late and the other 1 day early. well i dont know what to do, i really think that i could be pregnant but i’m thinking of making an appointment to get an ultrasound after christmas cause if i am, i’ll be about 12 weeks on christmas day. so i just wanted to know how far along were you when your stomach got hard? and any suggestions on what could be happening to me. thanks bye