A few years ago, my knee became "tighter", after a week I could not bend it. I got a lift to a minor injuries unit. Took me 10 mins to get out of a land rover, the lovely but direct nurse told me that I had a bursar (housemaid's knee). She gave me NSAIDS, I was fine the next day once the NSAIDs reduced the inflammation on the knee.
no expert at all
but iff its on the leg back you have possibly been using a muscle for retardation [slowing progress down hill] on the back off your leg excessively and the only time it engages is down stairs so it should heal naturally over several days
but purely a guess
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