Bakers Cist?

Anyone ever heard of a Bakers Cist? well Zayden has had a huge lump on the back of his knee for some time and so I took him to the doctor and they did all the tests and they said it was a bakers cist. I don't know if I really believe them though. We don't have the greatest doctors here in the great big town of meeker! So any advise or thoughts on the subject would be appreciated. They told me it should go away on it's own but if it's not gone in three months to bring him back in. it has been two months and it is not getting any smaller. I am not a worry wart mom, but i have to admit I am starting to be concerned...