I might also wonder if your cleats are too far forward on your shoes or if you don't use cleats, then are your feet too far back on the pedal. That longer distance from the pedal spindle to the ankle joint will mean your calf muscles will be having to deal with a lot of the power you put into the pedals.
Although it'll feel funny for more than several rides, moving your foot forward on the pedal might help.
Otherwise
Carbonfiberboy covered the better possibility IMO.