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 talked on his show about a hidden source of knee pain called a Baker's Cyst and how you can diagnose it. http://www.wellbuzz.com/dr-oz-general-health/dr-oz-hidden-sources-of-knee-pain-what-is-a-bakers-cyst/