Hi,
Your problem is likely to be a simple one, but you should still get yourself checked up for this. A large number of diseases could cause such numbness, the commonest ones being a lumbar disc lesion, conditions leading to peripheral neuropathy like diabetes, and Vit. B deficiency. Pain around the back going down the leg, numbness of the leg or foot and tingling or itchiness are all symptoms of a disc lesion. Most disc lesions are minor and can be handled with simple exercises, lumbar belts, steroid injections, non steroidal anti-inflammatory injections, etc. Some cases may need surgery. Peripheral neuropathies need to be well investigated to find out the cause. The commonest cause of peripheral neuropathies is Diabetes mellitus.You should consult your health provider or a neurologist if possible and get yourself checked up, you may need a CT or MRI of the brain and tests for muscle conductivity and nerve velocity, etc.

You may consult these sites for more info:

http://www.mayoclinic.com/health/herniated-disk/HD99999
http://www.ninds.nih.gov/disorders/peripheralneuropathy/detail_peripheralneuropathy.htm