Sudden Numbness On The Leg, Pain In The Knee And Feet, Difficulty In Standing. What Could Be Wrong?

Hello.. After working all day in the office (sitting mostly on a chair), when in the evening i walk to the bus stop just a kilometer away from my office.., in the midway, first my right leg upto the foot starts feeling numb.. later same happens with left leg, but i can still walk! then on the bus stop, if i am standing and waiting for the bus, my heels would start aching, numbness would increase, and it would become difficult for me to stand still, i will have to continuously shift my weight from left to right foot. then If after standing there for about 20,30 minutes and i get sit down on a bench or chair, i feel a sharp pain in my both knees. reaching home when i rest, everything goes away and i start feeling healthy again. what is the problem with my legs and heels?? Please help and guide!