Pain behind left knee when I bend it and it hurts to lay on it?

Baker's Cyst. Pain behind the knee with fullness and pain suggest a possible baker's cyst. A baker's cyst is a natural outpouching of the knee joint that can get full of knee joint fluid and can make it difficult to move the knee. This should however be evaluated by an orthopedic specialist for a proper diagnosis.
Get examined. There are many causes of knee pain that can be fixed or improved. See your orthopaedic surgeon and get the right diagnosis first.