Bring a large pot of lightly salted water to a boil. Add macaroni and cook for 8 to 10 minutes or until al dente; drain. While macaroni is boiling, fry bacon and place on paper towels to drain. Cut into bite size pieces. Melt margarine in a large saucepan over medium low heat. Stir in flour, then tomatoes and wait for sauce to thicken, stirring occasionally. When thickness is to your liking, stir in cheese until it has melted. Stir in cooked macaroni and bacon and heat through. Serve hot.