Your CT scan notes a large right posterlateral herniated disc at L5-S1 also causing some central stenosis and bilateral foraminal stenosis at the same level. There is mild degenerative changes of the L4-5 level.
This can compress the S1 nerve on the right or the L5 nerves on either side. The Si nerve would cause pain while sitting more than standing and the L5 nerves on either side would cause more pain when standing relieved with sitting or lying down. Since you note only left sided leg pain, the right-sided herniation at L5-S1 should not be causing symptoms.
Diagnostic injections can point to the diagnosis. See the section under injections on the website for more information.
Dr. Corenman