Bump/lump under knee?

I'm 17 and today I found a decently hard bump/lump (kinda soft) about the size of a quarter under my right knee. It's only on one leg. when I press on it, it almost feels like it moves a little bit. it feels a little sore only when I press on it. It doesn't hurt or anything when I am walking. if it is useful information, I have started running the past couple of days, so it may be exercise induced. what is this bump thing??? what should I do to make it go away? maybe it's that osgood-slaughters (sorry for the spelling)? I don't know. any input would help. THANKS