The area behind the knee has lots of structures, such as various muscles, nerves, blood vessels, and fluid filled sacs called "bursa".
It's possible that in inflammation
of the bursa "bursitis
" is rubbing on the tibial nerve
causing the pain. If this is the case, rest and ice (nore more than 15 minutes at a time) can help, as well as ibuprophen if you can take it.
A doc can evaluate further.
Answer assisted by Ken Starnes, Medical Student