My girfreind is having pain behind her knee at the caf muschle it burns sometimes and hurts what could it be?

Bursitis?: 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.

Bakers cyst: Many potential causes, was there an injury? Among the more common causes of posterior knee pain is a bakers cyst, which represents a herniation of posterior capsular tissue between two tendons at the back of the knee; other considerations include a posterior horn meniscus tear, popliteal aneurysm,even a deep vein thrombosis. Consider being seen for diagnosis, mri may help.