I would suggest discussing your concerns with your physiotherapist. Your symptoms may still be arising from your lower back such as a lumbar disc bulge which can readily change positions so that during CT scan, when you are lying down, your disc may not be bulging as prominently. Your physiotherapist will know if this is likely or will look to other potential sources such as the hip (e.g. labral tear etc.). You may be interested in checking out the above links for some detailed information.
Hope that helps,