What could be wrong with my knee? I fell from about 15ft straight down onto both of my feet, and bent my knees absorbing the shock. My right knee seemed to take most of the beating. It has been extremely stiff and has some tenderness, though I can move it

Microfractures. 15 feet is a big deal. There may be a fracture or a torn meniscus or ligament in the knee. I would have it evaluated and imaged.
This . This is a significnat injury and it is very likely that you have some damage to your knee. X-rays are clearly indicated but oftentimes are not too informative since they only show bone. Depending on your clinical picture an MRI or a ct scan could be needed. Do not ignore this injury.