>"EDIT: The only other possibility is that I think I found out that my region wasn't Michigan and changed it a while ago, but I'm not sure."
Is the most likely (and probably only) explanation. If your 3DS wasn't set to Michigan when you start your game, then the Scatterbug you catch in your game will not be from Michigan, but rather of the region you had set it as. Changing the region later does not count, so you're stuck with the Modern Vivillon now.
The solution to this though, is that you restart the game with the 3DS location set as Michigan. Then you would be sure to receive a Polar Vivillon.
Hope I helped!