The physio at our pre-natal classes said that there are ligaments at the back of the pelvis, joining the two hip bones to the tail bone. Some women experience pain here because these ligaments are looser during pregnancy. Some women experience groin pain for the same reason (at the front where the ligaments join the two hip bones together where your pubic bone is). I guess it is important to sit comfortably, not putting too much pressure on this area, and maybe ask your physio-OB for some exercises you can do to strengthen the muscles and help support your lower back.

Good luck!