<s>[INST] I had an arthroscopy of my knee 3-1/2 weeks ago and a meniscal tear shaved off. In the last couple of days I have noted a lump behind my knee on the back side of my leg. It feels about the size of an egg. Could this be an infection, blood clot, Baker s cyst. It is warm to the touch. [/INST] Hi, dairy have gone through your question. I can understand your concern. You have history of meniscal injury.  It can be some hematoma formation or abscess or backer cyst.  You should go for ultrasound study and if needed to go for fine needle aspiration cytology.  It will give you exact diagnosis. Then you should take treatment accordingly. Hope I have answered your question, if you have doubt then I will be happy to answer. Thanks for using Chat Doctor. Wish you a very good health. </s>