Most lower back pain is more innocuous unless you are busy trying to overcome it or sleep through it. The reasons for it are simple, and multiple. Your posture changes as your pregnancy progresses, shifting your center of gravity and increasing the weight you are responsible for bearing. Release of the hormone Relaxin allows joints to become more flexible, which is crucial to delivery, but it also decreases the stability in your lower back during a time at which it is under some stress. Abdominal muscles stretch to allow space for your baby's and body's growth and can no longer stabilize the pelvis, leaving already overtaxed muscles to do the job.
Circulatory changes cause even more pressure and pain.
Simple good sense will show you the ways out of all this suffering. Maintain good posture. Do not force your body into a straight line, and do not stand sway-backed. Wear sensible shoes, no matter how bleak you find them. Avoid standing for extended period of time, but when you do need to stand or sit for a long stretch, do so with one foot on a low stool, switching feet every once in a while.