What Causes Sensation Of Warm Fluid Running Behind Knee?

recently i've been getting a sensation like warm water running behind my kneecap. it happens intermittently and with no real provocation. i haven't had any recent injuries, but i was told long ago that i have tendinitis in both knees. i'm only 35, so i don't think it's an age thing. and there's no pain, just the warm sensation. any ideas?