A swelling that is prominent at the back of your knee could be a Baker's cyst or popliteal cyst. Baker's cyst or popliteal cyst might disappear sometimes on its own. Best Home Treatment for Bakers Cyst or Popliteal Cyst Would be Following the R.I.C.E. principles.
Dr Oz explained how something called a Baker's cyst could be the real reason for your knee pain and how to be sure. http://www.recapo.com/dr-oz/dr-oz-advice/dr-oz-is-a-bakers-cyst-causing-your-knee-pain-self-test/