The size, and weight, of your growing uterus is another factor, and, compounded by the fact that your abdominal muscles are stretching to accommodate your bump, and your centre of gravity has changed. It is no wonder your back aches!
Sometimes your uterus presses on a nerve, making the problem worse, and because your ligaments are stretching to accommodate childbirth, other pains could accompany your lower back pain when you sit, get in and out of the bath, or perform any other simple tasks. If you have existing back problems, you are more likely to have back pain during pregnancy, but most women will have some pain at some time.
Exercise and good posture are too important ways you can help to prevent and treat back pain during pregnancy, as is staying off your feet as much as possible. Avoid heels, and try not to lift anything heavy. Heat, cold, or massages may help, as can lying on your side when you sleep, but do not take any over the counter pain medication without first speaking to your doctor.