At around 12 weeks your uterus is starting to rise out of your pelvis and your organs shift a bit, so it’s normal to feel some pressure or discomfort when you sit. I remember propping a cushion behind my back and changing positions often. Gas and indigestion can make the sensation worse too, so try eating smaller meals and staying upright after eating. As long as there’s no severe pain or bleeding, it’s usually just things stretching, but if you're worried do mention it to your doctor for reassurance.