Preheat oven to 350 degrees F. In a large pot of water, boil noodles according to package directions. Drain water. Set aside.
In a medium saucepan, add Rotel tomatoes, chopped onion and Velveeta cheese. Cook over medium-high heat, stirring constantly until cheese is melted.
In a large bowl, add cooked chicken, cream of chicken soup, sour cream, chili powder, and cumin. Mix to combine. Slowly add in melted Velveeta mixture, and the cooked noodles. Stir to combine.
Pour mixture into a 9x13 baking dish. Spread out evenly. Top with shredded cheddar cheese. Bake, uncovered, for 25 to 30 minutes, or until hot and bubbly.