Bring a large pot of water to a rolling boil. Cook pasta until al dente, according to package directions. Drain and keep hot.
Heat olive oil in a large, non-stick skillet until hot. Add bell pepper and saute for 5-6 minutes or until beginning soften and brown. Add shrimp and sprinkle with Cajun seasoning. Cook 4-5 minutes or until shrimp is pink and cooked through. Remove from heat.
Meanwhile, heat butter in a sauce pan over medium-high heat. Add the garlic and cook for 30 seconds, stirring constantly. Add the flour and whisk constantly for 1 minute. Slowly add the milk, whisking constantly. Add thyme, oregano, salt, and pepper; whisk every minute or so until bubbly and thickened.
Pour the thickened alfredo over the pasta and using tongs, combine to coat the pasta with alfredo. Serve hot pasta with peppers and shrimp on top.