What Causes Pain In My Ankle While Walking?

i am having new onset pain in the back of my right leg posteriorly below the calf and above the ankle. It is a sharp pain when walking, not every step but when I step a certain way (how I usually walk). If I walk carefully I can avoid it. Each time it happens it is greater pain and after about 3-4 times it affects my ability to stand. After a rest period it starts all over. I have had no recent injury and i work at a desk, sitting most of the day. I wear flat shoes usually.