Hi, I am a 17 year old female and I have a small hard lump in my right knee inside of my leg. It is a little bit right to the right kneecap. It is like the size of a small marble ball, maybe smaller. It doesn't show but you can easily feel it moving around when I bend my knee slowly. It doesn't give me pain. I have this for quite awhile. I have never had surgery or any damage to my leg. I noticed that my friend has the same thing on the same place but her's does not move around like mine. What could it be? Do I have to worry about it?