It could very well be what is called a "bakers cyst".It is formed when there is some sort of major inflammation inside the knee.or it can also be formed by a small tear in the tissues of the knee.You might want to see your doc regarding this.Do you have any pain i your knee yet?There are some threads on this in this board or it was the other board at the top of the list hear where you can ask general questions.But if you just did a search on bakers cysts, there is alot of info on them.I have a large B cyst behind my R knee and there have also been others who have posted regarding these here.Just do the search and you should find them with no problem.after going through those, if you have any other questions, please post and i will try to answer if I can.this is just one possibility for your lump.Marcia