Leona,
This lump could be several things including a synovial cyst or a spinal cyst caused by a disc herniation.
Did a spine surgeon order the CT scan for you?
Have you seen a spine surgeon at all?
If not you need to see either an orthopedic surgeon that specializes in spines or a neurosurgeon that specializes in spines only.
No doubt an MRI will be ordered by the spine surgeon as well to help diagnose what is going on.
Good luck and please keep us posted on what you find out and the treatment the spine surgeon suggests.
Fran