Heat olive oil in a skillet over medium-high heat.
Season pork with salt and pepper then place pork loin in the skillet and sear until browned on all sides (about 2-3 minutes on each side). Place pork in a slow cooker sprayed with nonstick cooking spray.
In a mixing bowl, whisk together garlic salt, green chiles, brown sugar, red enchilada sauce and Diet Dr. Pepper. Pour over pork then cover and cook on low for 8-10 hours.
Preheat oven to 350 degrees F and spray a 9x13-inch baking dish with nonstick cooking spray; set aside. Remove pork from the slow cooker and place in a bowl. Shred pork and mix in 1⁄2 cup of sauce from the slow cooker; set aside.
Spread another 1⁄2 cup of sauce from the slow cooker in the bottom of the prepared pan. Divide pork evenly between tortillas, approximately 3.1 ounces each. Divide half the cheese evenly between the tortillas, approximately 0.5 ounce per enchilada. Roll up and place seamside down in the prepared pan.
Pour green enchilada sauce over the top and sprinkle on remaining cheese. Bake, uncovered, for 30 minutes, or until enchiladas are lightly brown and cheese is melted. Garnish with cilantro be