I rolled my ankle 2 weeks ago. since then ankle has mainly healed but my back of knee on the same leg has ached and is now extremely painful to weight bare. it is so tight i am physically unable to straighten it and cramps up. what have i done to it?

Knee: if your ankle is fine and there is no pain it will be worth wile taking some good pain relief and seeing a physiotherapist. if that doe not help , then se your GP

Clot or Bakers cyst?: The pain you are describing may simply be due to an associated knee sprain which may have happened at the same time. However this could be a blood clot in your deep veins or a Bakers cyst which may be leaking after the trauma. There is always a risk of an avulsion fracture in the knee joint as well. You need another examination with your doctor and possibly an ultrasound of your knee joint.

Leg pain: If your ankle is on the mend it may be some residual pain from your injury or from the way you were walking on it after the incident. Other possibilities include a strained knee or calf muscle, a ruptured bakers cyst (from the knee joint), or a clot. If it does not get better or it bothers you significantly your GP can sort it out.

Strain or clot: I agree with the other answers already listed. This is quite likely a muscular strain related to your original injury or to walking in an abnormal way to compensate for the injury. The most serious diagnosis could be a deep vein thrombosis (blood clot). It's impossible to tell which without an examination and maybe some further tests.

Maniacal injury: Sometimes you can also sustain a concomitant knee injury which may be masked initially by the more prominent ankle injury. Best to be examined and urgently if severe calf pain or leg swelling (to rapidly rule out clot)