Tight calf muscles are a common problem for runners. You'll feel a gradual tightening in your calf muscles when running. Sometimes the pain will go away while running, but then comes back after you stop.
Tight calf muscles are often caused by increasing your mileage too quickly and/or not stretching properly. Sometimes they are also due to a biomechanical problem that can be corrected with the right running shoes or orthotics.
One of the best ways to treat tight calf muscles is to see a sports massage therapist or physical therapist for a deep tissue massage. You should also make sure you're stretching your calves (without pain) and strengthening your calf muscles. Yoga is very beneficial for runners who are experiencing tightness. If you haven't already, go to a running specialty store to make sure you're wearing the right running shoes for your feet. If the pain persists for more than two weeks, make an appointment with a sports doctor.
Tight calves are a very common overuse injury, so make sure you're not increasing your mileage or intensity too quickly. You should increase your weekly mileage by no more than 10% each week. Also, make sure that you stretch your calves after every run.