What Causes Pain Behind Knee After A Meniscus Tear?

I injured my knee a week ago and the doctor states it may be a menisucs tear. I'm doing the RICE and taking muscle relaxers and pain meds. It doesn't seem to be helping and I keep getting uncontrolled muscle spasms in my knee. My leg pulls back until my knee is bent and it's extemely painful. The pain seems to be behind my knee and radiates around my knee afterwards. It is unbearable and I can't seem to find anything on line when reading about this kind of injury. What could be causing this?