Preheat the oven to 375 degrees F. Spray a 12-cup muffin tin with nonstick spray and place in the oven while you make the batter.
In a large bowl, combine the cornmeal, baking powder, sugar, red pepper flakes and salt. Stir to combine. Add the milk, oil and corn and stir until just combined. Stir in the Cheddar, onions and green peppers and stir just until incorporated.
Carefully remove the hot muffin tin from the oven and ladle the batter into it. Return to the oven and bake until a toothpick comes out clean, about 15 minutes. Let cool 5 minutes. Remove from the muffin tin and serve.