I sometimes get knee pain which can be cured when I go back to work and climb ladders or flex my foot like a ballet dancer.
Seems that there's umpteen ligaments holding the whole shebang in place. One continuous action can make one set of muscles so strong that they pull the knee out of place. You can strenthen the opposing muscles by special excercise or gain temporary relief by working the existing ones, however this will always cause the pain to return.
As our joints age we lose some of our flexibility & the synvial fluid in the joint causing pain because the joint surfaces get closer together. Running (or walking) causes a pumping action which encourages the joint to produce more fluid and spread it around. The joint space fills up with this fluid so the the ends of the two bones gets a little further apart reducing the pain.
Damselfly
ps. do you get enough fluids? dehydration can cause this to be worse.