Hello, Thanks for your query. Without a complete evaluation I may not be able to confirm the exact cause .It is possible that you have been suffering from Bakers cyst which has ruptured. A Baker cyst is swelling caused by fluid from the knee joint which may have protruded to the back of the knee.Osteoarthritis, cartilage tears, rheumatoid arthritis any of these conditions can lead to the formation of this cyst. There can be protrusion of fluid down the leg between the muscles of the calf or rupture of the cyst. Diagnosis can be confirmed by examination and radiologically. I do hope that you have found something helpful and I will be glad to answer any further query. Take care