BUT: until I and my body really figure out what the problem is, I've found that if I do this careful stretch I learned from biomechanist Katy Bowman (Restorative Exercise™), the tension and stiffness in the knee dissolve, and I can live on to figure it out another day.
Here are instructions from Katy's blog: "Lie down with your belly flat on the floor. Reach one leg out to the side without bending the knees! Relax your head and neck on your hands. Try to bring your leg up until it is at 90°. . . If you have a hip replacement, only stretch the leg that has not been replaced."
For DETAILS AND MODIFICATIONS
(along with a lot more babbling on my part) see: http://www.alignmentandmobility.com/blong/stiff-behind-the-knee