Ice that knee down and check your saddle height. Depending on where the pain in the knee is, it could be as simple as the saddle a CM too high or low. If your hips rock, then it's too high and if your leg isn't at full extension with your heel on the pedal, then it's too low.
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. “He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you.”- Fredrick Nietzsche
"We can judge the heart of a man by his treatment of animals." - Immanuel Kant