In a bowl, mix the shredded Cheddar cheese, green pepper, red pepper, cilantro, and tomatoes. Season with chili powder, cumin, and salt. Cover one side of the chicken breasts with enough cheese mixture to coat. Roll breasts over mixture, and secure with toothpicks.
Place the chicken breasts in a slow cooker. Pour in the remaining cheese mixture. Cover, and cook 3 hours on High.