How To Heal From Paining Knee?

My left knee has been hurting for 3 days now on the lateral side. Mostly it hurts when I sit still or move it out of the straight range of motion; for example walking up steps I'm fine but getting out of a car (which involves bending my knee out of line) hurts. It hurts most when I sit still. Also, I have this constant feeling of weakness as if it may give out on me. The only thing I can pinpoint is three days ago at cheerleading in our dance I have to fall from a standing position to my knees very fast and I land hard. Please help!