Where in your calf exactly is the pain. If generally all over then as said lack of conditioning. However if halfway down you feel pain when it is stretched, that's where the muscle becomes the Achilles Tendon. Possibly you strained that originally and are now feeling the problems associated with scar tissue. Often feels like a lump right in there halfway down. If the latter be prepared for a long haul rehabilitation (years) and accept that it will always be a weak spot.