Hi, Most common swelling behind the knee is the Baker s cyst which is a fluid filled cyst from the soft tissue behind the knee. It is also called as Popliteal cyst. Most common cause for the cyst is knee injury commonly seen in sports. Long term wear and tear of the knee joint also leads to cyst formation. I suggest ultrasound to rule out blood clot or tumour. Baker s cyst resolves on its own usually. Sometimes it rupture into the knee joint and fluid may leak into leg causing severe pain in the calf. Painkillers helps to reduce swelling and pain such Ibuprofen and Paracetamol. The fluid in the can be aspirated through a hole by surgery. Weight reduction and rest to knee joints will help to relieve the symptoms. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Pradeepraj, General & Family Physician