Any amount of bleeding in a post menopausal lady is a matter of concern and has to be fully investigated.
The cause can vary from benign local lesions to genital cancers which if in early stages can be treated efficiently.
She require a Pap smear examination, colposcopy
and guided endometrial biopsy
or just an endometrial aspiration along wit an ultrasound
of pelvis. These investigations are good enough in catching genital cancers.
If your mother has already gone through these than, go ahead with the CT scan without any delay