<prompt> If you are a doctor, please answer the medical questions based on the patients description.I have a pea-sized bump in the back fold of my left knee. It can t be seen, but you can feel it under my skin. When I get chills, stand too long, or sit too long, it causes great pain. I was once told that it s probably just a hair follicle on a nerve. It s been here for eight years and has gotten a little bit larger over the years. It is hurting now because I m sort of cold.  <response>: Hi, Dear,Welcome to Chat Doctor. Based on the facts of your query, You seem to suffer from-Sebaceous cyst chronic furunculitis or could be Varicose Veins with Short Saphenous Incompetent valves. Pain on long sitting standing too long, may suggest DVT association with thrombophlebitis. Chills with it favor the thrombophlebitis factor with it. Chronicity of the lump 8 yrs makes it more of Dormant Sebaceous Cyst with recent activated re-infection.USG Color Doppler dual Imaging of the lump, would fix the Varicose cause of it. Contact with a Followup Premium question to ME. Will appreciate your Hitting thanks and writing excellent review comments to help needy patients like you. suggests Good Day!! Chat Doctor. S. Genl-CVTS 