I developed intense knee pain one day after running. I had been sick for a few weeks and went back to my regular routine and after my 20 miler of the week I found I could no longer walk up the three steps to get into my house. It isn't that it hurt so much but that my knee felt weak and unstable. I sat and layed in front of my steps for a few hours until I could get up enough to unlock my door. In that time, I decided I was weak and just needed to cut back mileage for a couple of weeks. I was wrong. It's about ten years later and I still can't run more than 9 or 10 miles without a week of prep first. I saw doctors, PT and hippies. Pain relievers and muscle relaxers helped but made it hard to keep my cadence. PT gave me stretches and a resistance band and told me to do exercises for three weeks before trying a short run. The hippy gave me some RC that was kind of like acid. I wish I had just listened to the PT. The IT band stretches ended up relieving my pain and made it so I could reliably go up stairs again. I recommend seeing a PT or sports med guy and doing what they say. Otherwise, for me it was IT band stretching and switching to zero drop shoes. Good luck, get healthy and stay there!