I've been sitting here, trying to think of someone, anyone, of my acquaintance who had a posterior knee problem. I'm the only one I can think of. My problem was a slightly torn meniscus, torn by one of the posterior knee tendons, I forget which one. 15 years ago, maybe. But i knew exactly when I did it and how. I massively overstressed that tendon. I've torn hamstrings too, but they aren't behind the knee. I've done a good bit of crazy stuff.
All that said, don't wait for the fitter. Just lower your saddle now, try stretching a little, and see how that goes. My guess is you might have it resolved before the appointment. Not that you shouldn't go even if you feel perfect. There's always more to learn.
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Results matter