Why is my leg sore and numb?

To properly answer and diagnose what is causing your pain, any health care practitioner would need a little more information. Such as;
- do you have back pain accompanying the leg pain?
- how did the leg pain start? With a traumatic event I.e.twist or fall, gradually with increased exercise?
- is the pain down the back, side or front of thigh?
- what aggravates the pain? Movements of leg, hip or back? Neck movements? Cough or sneeze?  pain all the time?
- quality of pain? Sharp, dull, electricity?

These are just a few questions I would like to know before diagnosing your condition.
If I had to take a guess based upon the info you have provided me, I would say it could be one of  two things.
1) piriformis syndrome with a entrapment of the sciatic nerve
Or 2) a lumbar disc herniation with spinal nerve pain .

If you do not have any history of back pain or injury, the first condition would be more than likely.
Piriformis syndrome appears to occur due to repetitive overloading of the hip musculature of the gluteal region. As the sciatic nerve passes closely across and/or through the piriformis muscle, the nerve can commonly become entrapped or squashed, which either directly irritates or starves the nerve of oxygen, particularly during exercise and exertion of these muscles.
If this is the case I would suggest seeing your health care professional about advice on stretching and how to manage this condition. Have them assess your lower limb function to identify any flaws in your movements and stability. You may also benefit from a course of physical therapy.

Dr Andrew Lim has comprehensively answered this question and I would only add that numbness is generally a neurological symptom meaning that nerve structures are being compressed or impinged upon at either their source or along their pathway.

A thorough orthopaedic and neurological history and examination is then required to assess the motor and sensory function of the lower limb nerves, muscle stretch reflexes and nerve root tension tests.

Registered chiropractors, osteopaths and physiotherapists are able to then combine the health history and examination findings together and determine what type of imaging might be indicated to help with the final diagnosis.

It sounds as though you have referred leg pain that is acgtually coming from your back.  IMost back pain happens when we sit and slouch and from all the bending and stooping we do.

I suggest consulting a McKenzie trained Physio who will show you movements to avoid but importantly the key pain relief exercise to reduce the leg pain and centralise it to your back. 

Google: McKenzie Method
Google: centralisation of back pain
Hope this helps

I would only add to the above by advising to think about what environmental factors are contributing to the above mentioned posibilties and how they aggravate the symptoms. Ie prolonged sitting, repitition from walking/overuse etc. 

Numbness is a sign of nerve involvement.
The most common site for nerve to become compressed is in the lower lumbar spine. In fact, the inside of the lower leg is supplied by the L4 nerve root.
Patients often find this difficult to swallow, but many patients with leg symptoms caused by the back don’t actually have back pain. It’s quite common however for these patients to have a history of back pain and stiffness possibly over a period of several years. Typically as the back condition becomes more severe, back symptoms may be overtaken by leg symptoms.
When there is numbness involved in the lower leg it’s very important to assess for muscle weakness and atrophy or wasting. This is a simple matter of applying repeated muscle testing and measuring the limb and comparing the findings to the opposite side. It’s important to establish baseline findings in this area because long-term nerve root compression can lead to other problems.
The lower back is a very common cause of referred pain and neurological symptoms into the hip thigh and lower leg. The first step would be to have a comprehensive history and mechanical evaluation performed by a qualified McKenzie Method therapist.

Hope you have found this response informative and I hope your issue is resolved soon.

Reagrds,

Brett