What Causes Weakness And Stiffness Of Knee?

Hi, I would like to ask about my knee. It feels very full and stiff, I noticed this sensation 4 days ago and gave it a few days to subside but it hasnt. It feels unstable and feels like it I kneel or step or twist wrong that my whole knee will dislodge. And it feels weak. But the kicker is it doesn t hurt. Just feels full, weak, stiff. Can t bend it without a LOT of pressure