Why does shin splints hurt when you are running and tighting up in gym class?

Likely normal: Shin splints occur from micro trauma where muscles connect to the bone, actually tiny tears in the muscle. Normal after intense workouts, try backing off on the intensity and frequency and let your body heal. Could also be muscle cramps which are often due to dehydration or electrolyte problems. Stay hydrated with a sports drink such as Gatorade. Aspirin or motrin and muscle massage may help.