yeah, I was thinking baker's cyst too and twopack beat me to it!
feel along the back of your knee (where it bends) and if there is
a small bump that isn't usually there, you got one.
I get them and sometimes bilaterally and they look like golf balls under the skin but no one has ever given me a good explanation other than its
the fluid leaking from the front and it has to have somewhere to go.
It will self absorb and go away
Any chance it's a baker's cyst? They're pretty common and cause a feeling of tightness behind the knee. The pain with that usually increases when your knee is fully flexed or extended.
You can have spasticity that causes stiffness along the side of your knee (I'd say maybe typically along the sides, but maybe some display behind their knee). My ankle gets twitches, too, but I have no toes that shake.