Heat the oil in a large, heavy skillet over medium-high heat. Add the pork and cook about 5 minutes, stirring often, until lightly browned and no longer pink. Remove the pork with a slotted spoon and set aside on a warm plate.
Wrap the tortillas in foil and warm them in the oven for 6 minutes.
Add the green pepper, onion, chili powder, salt, and pepper to the skillet and cook over medium heat for about 5 minutes, stirring often, until the vegetables are softened. Stir in the chopped tomatoes, add back the cooked pork, and continue cooking another minute.
To serve, spoon some of the pork mixture onto a warmed tortilla, add a dollop of salsa and sour cream, and fold up the bottom and sides to make a fajita. Repeat with the remaining tortillas.