You'll have to see a doctor to find out what this is. Something called "Baker's cyst" - which is basically an accumulation of fluid - can cause swelling and pain in the back of the knee. Sometimes problems with the kneecap cause referred pain to show up in the back of the knee. I'm sure there are other possibilities as well.
A doctor's visit should be able to clear this up. Good luck!
Thank you very much I will go get it checked out.