Hi and thanks for the query, its important to know for how long you have been having this pain. Any notion of trauma or not? does it affect only one knee or both knees? Accompanied by swelling? What aggravates or calms the pain? Any history of knee surgery? These questions are important to be considered. A physical examination shall then rule examine the size, tenderness, signs of inflammation, the nature of the posterior aspect of the knee, to ascertain a possible Baker s or Popliteal cyst. I suggest you get a proper consultation from your family physician. An ultrasound might be needed at some point, or aspiration and analysis to ascertain the fluid contents. Thanks and best regards, luchuo, MD.