The same thing happened to me my freshman year of college. I was on a bus and had put my knees up on the seat in front of me the entire time. My feet had fallen asleep and I noticed afterwards that a spot on my leg, just below my knee, stayed numb. As I remember it was about the size of my fist and it was in the spot that had been pressed against the seat in front of me. I thought it was weird but never went to the doctor. That spot on my leg stayed numb for several months before it finally went away on its own. It was exactly how you described it - numb at the surface but I still had feeling underneath the skin. The only thing I can think of is I somehow damaged a nerve that eventually after several months cured itself. It was definitely a weird experience.