Knee joint is dependent on the ligaments mainly for it's stability (apart from other soft tissues like muscles). Your symptoms point to a ligament rupture (likely to be cruciate ligament). This rupture can be old which can surface after some time, even after a very mild injury. An MRI of the knee will diagnose it.
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These symptoms are typical of the ligament injury. When it gets dislocated than only it becomes painful otherwise it is painless. An arthroscopic surgery (camera surgery) would be able to repair it. You would need an orthopedic appointment, as soon as possible.
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