It sound like you have compartment syndrome. It's pressure build-up in a certain compartment of the leg. I am not a doctor but I am in the army, and I began to develop numbness in the back of my right calf and ankle. It wasn't pain but it was very uncomfortable to the point where when I ran my foot went numb as well and starts to drag. Sudden increase in excersice, especially running, is usually the cause. I just had fasciotomy, which is surgical treatment for it. You get an incision on the affected compartment and fluid, which causes the pressure, is released. You should be good to run again after a month or so. The problem with me was they cut over the wrong compartment (the side of my shins) so when I run, I still have the same problem!
I have a numb skin sensation on the back of my ankles and lower calves. I jog 4-5 times a week, not far, a couple of miles. My skin in that area feels numb all the time, and no pain. Any help would be appreciated.