I went for a 5k walk with the other half on Sunday and could feel a twinge in my left knee but not unbearable. Then yesterday I took my new trainers for a gentle 5k. I could feel the same thing at the start but then it went away and I was fine. I was really good and did a warm up and cool down walk and stretched after. Today I got up and it was definitely stiff and feeling 'fat' behind the knee when I bend it and it's got steadily worse throughout the day. Rest? Run through it?