<s>[INST] Last week I noticed a lump in the back of my leg, just below my knee. I don t know what caused it. It is not bruised. It s approximately the size of a golf ball, firm, and hurts when I press on it. Sometimes my calf and even foot feels tight. I do water aerobics 3 times a week, but I don t recall a sudden pain. I am 53 year old breast cancer survivor, so of course I m wondering how concerned I should be ... [/INST] Hi, Dear , Thanks for the query to My Chat Doctor clinic. I studied your query and understood your concerns. In my opinion-you suffer from-Popliteal cyst or /? Morant Bakers Cyst of the Knee.-Its due to the arthritis of the knee with cystic swelling thought the defect in the Popliteal fascia of the Knee joint capsule.-You need to consult an ER Orthopaedic-Surgeon who would Repair MB cyst or popliteal cyst. Hope this would resolve the issue and would help you to plan treatment. Wellcome for further queries in this regard. Good Night. </s>