What does it mean when your lower back hurts when you are pregnant?

Physics & nerve pain. 1) your center of gravity changes with weight and bulk being added to the front of your body, so you and your spine adjust to accommodate thus. 2)your pelvis enlarges and the sacro-iliac joints (where pelvic bones and spine join) are stretched. 3) direct internal back pressure from enlarged uterus. These factors can all activate pain sensors in your sensory nerves causing sensation of pain.