hi..thanks for your query..
From what you have mentioned, I feel you have developed a soft tissue
growth behind the knee area.The soft tissues swelling mentioned might be a cyst/ a lipoma
or any other extra growth of one to the other components of the soft tissue.If the skin around is also looking different or there are associated complaints of pain /tenderness you should consider the possibility of a superimposed bacterial infection
In either way, it is extremely important that you visit a dermatosurgeon who would help you find out the exact nature /cause of the swelling and treat appropriately .It might require a few investigations and post which a complete surgical removal of the extra growth should happen.
Hope this helps..Take care!