Running with a cramp can be really unpleasant. Sometimes the cramping will stop while you are running, but if the cramp doesn't subside after a few minutes, you may decide to slow down. Runners also use many psychological techniques to continue on through discomfort such as revisiting your motivation for running, visualization of the finish line, mentally focusing on your gait, aiming for the next landmark, etc. Use whatever techniques work for you to distract you from your discomfort.