Heat butter in a large skillet over medium heat until melted, 3 to 5 minutes. Stir in onion and green pepper; cook and stir until softened, about 5 minutes. Add shredded pork, water, and fajita seasoning; cook and stir until heated through, about 5 minutes.
Place tortillas in a skillet over low heat until warmed, 1 to 2 minutes. Remove tortillas from the skillet. Spoon shredded pork mixture into tortillas; top with lettuce, tomato, Cheddar cheese, salsa, and sour cream.