Cook pasta according to package directions. Meanwhile, in a Dutch oven, melt butter over medium heat. Stir in flour until smooth; gradually add the milk. Bring to a boil; cook and stir for 2 minutes or until thickened.
Reduce heat to medium. Stir in the cheese, salt, mustard and pepper. Cook and stir until cheese is melted. Drain pasta; stir into cheese sauce. Cook and stir for 3 minutes or until heated through. Stir in tomatoes just until combined.