The type of pain your describing is likely to be a "Neuropathic Pain" as you have numbness
.. if u really know what numbness means
Do you have any tingling
Does your pain gets aggravated on lying down or it improves?
Does your pain radiates to your leg or foot?
Since your pain worsens when you sit down its more favourable for a mechanical pain.
If you are sure you have numbness ... than you require an examination by a dr and an imaging study of your lower spine which will help to rule out disc problems and other conditions too if any
And to add to your surprise this type of pain is not due to inactivity. This can be a result of overactivity.
Dr Anshul, MD