Heat oven to 350. Boil chicken breast for 20 minutes. Drain and cool. Cook spaghetti noodles al dente. Rinse and drain. Place in 9x13 baking dish. Cube chicken and velvetta. Place in dish with noodles. Sauté mushrooms, onion and bell pepper in butter. Mix with noodles. Stir in rotel tomatoes, cream of mushroom soup, milk and black olives. Bake covered for 20 minutes. Remove and stir. Sprinkle shredded cheese evenly on top. Bake 10 more minutes uncovered. Serve with garlic bread and/or side salad.