Not sure what is wrong. I finished a climb without falling or hitting my knee. Then fifteen minutes later I started to notice swelling and stiffness. In four hours it was the size of a grapefruit and I could not walk. My knee was very stiff; I could not bend it and it was very painful to put any pressure on it. I went to the ER the next day and they initially diagnosed me with cellulitis. I finished the ten days of antibiotics, and the swelling was mostly gone and I would walk, but it was still present. I went for a three mile, flat hike to test it out and when I finished the swelling went up and it was painful again. I have seen two doctors since. Both are unsure what it is and have ordered blood work to test for lyme and various biomarkers.
Overuse injury in the end. No clear diagnosis. Stretching hamstrings and strengthening quads.