I took your Tanner stage test and I'm at stage 4. My torso is very disproportionate to my legs. What is quite weird is that I have the same leg length as all my friends who are taller than me. I know the torso usually grows in the 4th stage of puberty. I have started to have lower back pain. Do you have growing pains from your torso?
Lower back pain is more common in girls than in boys during puberty. While not unusual, you should have it checked out by a medical doctor or chiropractor. Problems like scoliosis or uneven length of legs might be causing the problem and these can be treated. More often, nothing in particular is found and it will fade on its own after a while.