I have soreness and inflamation behind both knees. what could cause this?

Tendonitis? muscles?: While its hard to tell the exact reason, it would most likely be a muscular strain or possibly some tendonitis affected the area. This could be secondary to overdoing certain exercises or activities also, you want to make sure you dont have phlebitis? Try ice, nsaids and mild stretching. If pain doesn't improve or worsens see an md.

Sore knee: It could muscle sprain, inflamed tendon or baker's cyst . Examination would confirm the diagnosis.