I have had that problem for a couple of months now. It is probably your ligaments. When they get too loose, they get strained, and it hurts. Plus, the weight of the baby pulls on them, and some are attached to your lower back, so it creates pain.
You can go to a chiropracter and it may help, but really the best thing to do is to rest, stay off your feet as much as possible, and move your legs together when you move them. Like move both out of the car at the same time, etc.