Needs a look. This might be something as "simple" as a muscle cramp, possibly from dehydration. Or it could be something much more urgent like a deep venous thromboembolism (blood clot, dvt). Only an examination and workup will be able to determine the cause. Is your calf swollen, especially hot or cold, or hardened (as opposed to soft)? If so, you should probably make your way to your local emergency room.
PVD or DVT. Peripheral vascular disease, DVT or leg cramps dye to any electrolyte disorder.