I have a pain at the back of my knee for a few days and know it seems to be travelling down my calve to my ankle it doesn't feel like muscle?.thanks

Several: Pain in back of knee traveling down leg w/ no injury mechanism in your age could be several things. If it worsens or continues, get examined. Cysts can start in back of knee and as they " leak" fluid can cause discomfort down leg;calf ( gatroc)muscle strains can cause pain down leg( even though u feel its not muscle)as well as a nerve irritation(. Esp.If burning or " pins/ needles type pain); etc.

Pain: As dr trettin said , this could be several things. An exam will answer your questions