my leg is numb?

Sciatica nerve may be an issue..................

May be from shin splints..........

TTS compression of the tibial nerve

Check out if you have Reynaud's syndrome: "Raynaud's (ray-NOSE) disease causes some areas of your body — such as your fingers and toes — to feel numb and cold in response to cold temperatures or stress. In Raynaud's disease, smaller arteries that supply blood to your skin become narrow, limiting blood flow to affected areas (vasospasm)."

DVT .  are you on one of those new type birth control pills ? you got a blood clot in your leg . it can move to lungs and give you a pulmonary embolism , to your heart and give you a heart attack , or to your brain and give you a stroke . you need blood thinners immediately . go to ER . 

we can't make diagnoses online, you must ask a physician; it could be a nerve damage or compression, or a circulatory problem, like clots, vessel damage or compression, or other completely different causes; compressions to vessels and nerves can be due to herniated disks, muscle swelling, scar tissue, compressive clothes or shoes, laying or sitting in wrong positions, and other causes.

That is odd. Possibly a circulation issue. What had you been doing beforehand?

If it does not improve with more walking, tell a parent. It is unlikely but possible that you have developed a blood clot.

O. Cold and numb may need the ER