The baby grows bigger in the uterus as the months go by, expanding the stomach. The bigger the uterus, the more weight it puts on your pelvic area and changes the center of gravity of your body. This makes you walk funny as you start walking with a much wider gap. This directly affects your posture, and as it changes, you might feel an ache in the lower back. You can try using a heating pad, massages, yoga, or stretching exercises to provide relief.