Severe Knee pain on a knee I injured 10yrs ago. Hurts to leave it still and move it. Tried Motrin and Tylenol (acetaminophen) for pain. Slightly swollen behind knee?

Get it checked.., This could be lots of things. Given the chronicity of this problem and your symptoms, you should see a doctor for an eval and X-rays.
See details. Then see an orthopedist or rheumatologist and have the knee properly evaluated.