Most women experience some changes in the bone structure of their pelvis during pregnancy due to the hormonal changes, particularly relaxin which causes the ligaments and joints to relax in preparation for the birth. Sometimes, the pelvic bones shift too much, stretching the ligaments and causing pain. Sometimes the bones in front will separate, which is extremely painful. Most of the time the bones settle down quickly, but if the settle in an awkward manner within the joint(and you have several joints that make up "the pelvic bone"--it's not just one rigid bone) then the pain can be pretty bad causing walking & sitting difficulties.
What treatment are they offering? I went through this with #2--pelvic separation--and it was pretty much rest and ibuprofen.