text: There are a lot of possible reasons why you are feeling this pain. It could be arthritis (osteoarthritis), a patella problem (patella is your knee cap), or a problem on the knee bones (distal femur and proximal tibia). I can see that you are a runner. This means that you put a lot of stress on your knees. If there is no trauma or you do not recall hitting your knee, then I can assume that it is osteoarthritis. Contrary to what others believe that osteoarthritis is for old people, it could also happen to young people. What I can suggest is for you to put a hot compress on the affected area. Stretch before running and most importantly, strengthen your leg muscles. This is important because the muscles help absorb the stress on the knee bones. You can do lunges or partial squats. Note that I put PARTIAL because if you do in fact have an arthritis, you should avoid bending your knees more than 45 degrees. If, on the other hand you recall hitting your knees, it may be your ligaments or the patella that is affected. That will need xray or MRI. Good luck with your knee problem!