[sensitivesoul]

It sounds like a tight tendon to me. Mine have been extremely tight (been stretching them lately but before this it's been years).

Find when I stretch them now i get a bad pain behind the knee and down the calf. If it is a tight tendon then there's nothing to worry about, just do stretches for them every day (they will hurt at first but that will go away over time).

Though the tendon might be torn because it did come on suddenly so i would recommend not to stretch them to much and just take it gentle.

It doesn't sound like to bad of a pull/tear so cold behind the knee should help along with slight stretching till it does fully heal.

After it does heal I'de recommend doing tendon stretches so you can loosen them up and the likelihood of this happening again is slim.

This link also suggests other ways that might help a torn muscle. It doesn't sound like a severe tear though so it should be fine.
http://orthopedics.about.com/od/spra.../ht/muscle.htm