It's been about a month since you posted. How has the problem developed? Do you still have pain? Would be very interested to know as a student physiotherapist. From the pattern and location of pain and the direction of impact I would suggest it was a tear of the posterior Wikipedia reference-linkcruciate ligament which is a ligament in the middle of the knee preventing the lower leg (tibia) sliding forward at the knee. Or possibly a Wikipedia reference-linkmeniscus tear?

Cheers,
Adam