Something happened (no, I don’t know what) a couple of weeks back as a result of which my left knee was horribly sore for a few days. Then the lingering part: Horrible pain trying to walk up stairs, and this part seemed to be actually getting worse.
Then I discovered: Although I can’t walk up stairs, I can run up stairs. No pain, no hassle. Just run up stairs. SOLVED!
And a day or two later, it feels a lot better since I haven’t been stressing it, I’ve just been running up and down stairs a lot.