I fell and hit my knee (pretty hard). i can move it just fine with no pain, but if the knee is touched it fells like a knife i am 41, in good shape, all around pretty healthy

It : It sounds as though (at minimum) you have sustained a pretty severe contusion and the persistent pain is likely the result of a bone bruise--these can cause severe sharp pain that persists for weeks, but are not serious. Assuming there was no twisting or angular component to the injury, which could have resulted in a cartilage or ligament injury. Depending on the force involved (and height from which you fell) you may also have sustained a fracture. I recommend ice, compressive wrap, and limit your activity until you can obtain an xray; which you should do as soon as possible.