How To Get Relief From Knee Pain After Twisting?

Hi, about 4 years a go, I twisted my left knee in school. I had alot of pain but no sound or poping was heared. I was taken off the knee as soon as I got to the nurese office. Anyways. Every once in a while, my knee will hurt again and some times become weak. What could cause the injury to cause pain when nothing was seen or felt by the doctor? I am currently 20 and 6'2".