Why sometimes i feel bit of cramp at my calf muscle?

No definite reason: Exact cause of muscle cramps is unknown, but the theories most commonly cited include:altered neuromuscular control, dehydration, electrolyte depletion, poor conditioning, muscle fatigue, new activity..Serious muscle spasm doesn't release on its own and requires manual stretching to help relax and lengthen the shortened muscle. Spasms and cramps can be mild or extremely painful.