hi i agree with the SI joint thing of alophysio. disc herniation should increase the pain in sitting i would say. but its also possible. Also the hip joint might be reason for the problem, the age is typical for degenerative problems, i.e. arthrosis. that she felt unwell might be caused by pain, or she was nervous and afraid that pain increases or something like that. GP should rule out and intestinal problems as well.
hope you will let us know more, soon.
cheers