Do you have any swelling - a slight pouching - behind the knee?
The account of your pain sounds like the popliteal cyst I had treated - synovial fluid collection in the back of the knee caused by a tear in the front of the knee. I had tightness (read pain) directly behind my knee, which was worse when I flexed my leg and whenever I extended it.
BTW, it's not a "real" cyst per se and was treated by my doctor by asperating the fluid with a needle from the front of the knee - don't look if you don't like big needles - and a cortisone injection.
Just wondered if you didn't get a tear in your knee last week with the fall that could be causing fluid leakage to the back of the knee.
It's a long shot but something to think about.
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