A sudden, sharp, pain in the thigh or behind the knee may be a torn hamstring, or possibly a tear in the posterior part of the meniscus. Alternatively, if you have sudden pain and instability after an injury, perhaps accompanied by a popping sound, you may have damaged a knee ligament. It could also be a ruptured Baker’s cyst. In these cases, whilst RICE (rest, ice, compression and elevation) is recommended initially, you should go to your doctor and have the knee clinically assessed.