I woke up in horrible pain just below back of my knee. like clot going down. hot rushed feeling. then stop. now feels like rip behind leg. what is it?

Torn calf muscle: Sudden onset of this tyepof pain is most likely from a cramp in muscle for leg during night and then a tear. Have to see a doctor soon for diagnosis.

Leg pain: Generically, it is leg pain, but the true diagnosis would be found by getting an exam and maybe some testing. Conditions could include a ruptured bakers cyst, dvt, thrombophlebitis, torn muscle, torn ligament, bad sprain, bad strain, etc. Best wishes.

Blood clot: Your initial impression of a blood clot needs to be ruled out by proper testing in the er. Please go now. Hopefully it is something less serious, but i would advise you to have a blood clot or deep venous thrombosis ruled out and treated so you don't risk throwing a pulmonary embolism to your lungs. Best wishes.