Why can’t I straighten my knee?

I haven’t had any leg injuries or anything, but my knees won’t straighten properly. My kneecap always sticks out. It is also painful to sit with my legs straight in front of me. I try stretching but I’m unable to do it because of the stretching pain. Is this just inflexibility or something more serious? I am a teen... show more I haven’t had any leg injuries or anything, but my knees won’t straighten properly. My kneecap always sticks out. It is also painful to sit with my legs straight in front of me. I try stretching but I’m unable to do it because of the stretching pain. Is this just inflexibility or something more serious? I am a teen girl btw.
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