In a large saucepan, bring 4 quarts of water to a boil. Add the rotini, reduce to a low boil and cook to the desired doneness, about 10 minutes, adding the tortellini during the last 6 minutes of cooking. Drain and transfer to a large bowl.
Toss the tomatoes with the salt and pepper in a small bowl.
While the pasta is still warm, add the tomatoes, Cheddar and oil and toss to combine. Serve warm.