The first answer is sound and probably the reason. Another possibility is that a nerve, particularly in areas of stress, ones that take quite a day to day beating, such as knee/elbow/ankles it can sometimes be a nerve that's started to protrude a little further than normal. I had a nerve on my knee that did just that. I could pinpoint the precise spot. Even the slightest knock, would literally floor me. With time, the nerve will recede, but it could take a few months. Try padding the area for protection from knocks. I concur that it did feel like a drawing pin. A large one.