Possibilities that may need to be considered include inflammations, bursitis, infections, degeneration with joint effusion, injuries to tendons/ ligaments/ internal
knee structures, neuro-muscular
causes etc. It would be best to have this evaluated by a primary care physician or an orthopedician for an accurate diagnosis and appropriate management. Meanwhile I would suggest resting the knee and limiting weight bearing. If the pain is more than mild, Tylenol may be helpful for pain relief.