What Causes Indentation On The Shin Bone?

This morning when I woke up my upper shin right below my right knee was hurting severally. The pain went away. But tonight after getting out of the shower I noticed an indention in the bone from where the pain was coming from. There's no pain there now. What could this be from? There's no previous injury and I haven't exercised this week. Yesterday was my day off so I did little walking and standing.