Hello, Anon. Thank you for writing.
Among the most common causes of pain on the (outer) outside of your knee are lateral cartilage injuries, including an injury to your lateral collateral ligament (LCL). The main cause of LCL injuries is a sudden impact or trauma to the inside of your knee, which might occur during an accident, sport, or sudden twisting motion. Have you injured your knee in any way (that you can recall)? There are, of course, other causes of knee pain. An evaluation by a doctor is needed to make a proper diagnosis, which may include an x-ray or other imaging.