I Am Female 84 Years Old, Weight 127, 5'3" And

I am female 84 years old, weight 127, 5'3" and in fairly good health. I had a knee replacement a year ago and that has healed well, no problem with that. This morning I woke up and the front of my shins looked dark, like they were bruised, but I have not bumped myself or did any other damage I would know of. I put on compression stockings and tried to stay off my feet. By late afternoon the color was just about back to normal but I am worried what could have caused this ?