Few months ago, after i ran for a long time i had pain in my legs muscles. and now the pain appears each time i start running. iam 20 years old. why?

Spasm? tear?: Could be technique, shoes, body mechanics, less likely referred pain from back. Might have had small tear in muscle, takes a long time to heal, re-traumatizing area. Might benefit from physical therapy; see md.

Shin splints: Try stretches and supportive running shoes. If not better see a sports medicine doctor

Lower intensity: Try to lower the intensity of your activity. You may start with waling and gradually increase the speed, till you can run without the pain. Have a healthy diet, exercise 30 minutes/day, drink plenty of water, have safe sex, no tobacco alcohol weed or street drugs. For nutrition: http://www.choosemyplate.gov/. Get HPV vaccine