What is this under my knee?

During my dance class I developed a strange random pain under my knee. Nothing like a bruise was there when I looked. A few hours later the pain stil

During my dance class I developed a strange random pain under my knee. Nothing like a bruise was there when I looked. A few hours later the pain still was there so I had another look and a random rectangular purple/red bruise has appeared. What could it be?

or:During my dance class I developed a strange random pain under my knee. Nothing like a bruise was there when I looked. A few hours later the pain still was there so I had another look and a random rectangular purple/red bruise has appeared. What could it be?

Tags:bruise,