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.
Baker's Cyst - A ganglion cyst or synovial cyst can occur in any joint in the body. They appear under the skin as a lump or bump and may sometimes present no other symptoms. The cyst is basically a soft sac filled with a thick, clear fluid. It's still unknown why cycts form so close to a joint or tendon | AidMyMeniscus.com