The other thing it could be is an excess of loosened ligaments. Pregnancy hormones cause the joints at the front of the pelvis to loosen to facilitate birth. In some women this also affects the hip joints making them unstable. Sometimes the knees are affected as well. I had a work colleague who used to have to go onto crutches mid pregnancy and then progressed to a wheelchair for the last six weeks or so.
http://www.whattoexpect.com/pregnancy/symptoms-and-solutions/symphysis-pubic-dysfunction.aspx