I hurt my right knee 2 hours ago, and I can t bend more it than 90 degrees. I hit it against the side of a stone wall while climbing down from it. It hurt just after, then went away, but since an hour agao, it hurts much more, and I can t bend it, it feels stiff, and I can t walk without having to keep my right leg really straight. Is it serious, what should I do? Thank you