It happened to me when I was playing in an exercise outdoor place, and I tried to do a sling flip on one of the poles, (not that kind of spin, there's been where I move around) i fell down and scratched my knee on the cement. It looked kind of like the picture you show me. So i'm guessing you got hit by some dirt on the cement using an alcohol wipe a little bit real hard on it.And it will probably go away