So I was just playing Pokemon Y. My Braixen (female) evolved yesterday and I just realized that its gender switched to male. How is that possible? Does Delphox have a 100% male gender ratio or something? Please note though- I'm actually not exactly sure when it switched- I just noticed it, but I think it was at evolution. I know that it's not a case of having mistook its gender when I first received my starter- I named it Aurora and even thought that I was lucky to have a girl Fennekin. So... can pokemon switch gender when they evolve? Is there a way to change it back?
TL;DR Most likely, you misread Fennekin's gender.
Delphox has the same 87.5% male-to-female ratio as its previous evolutions, and there's no known glitch that would cause it to change to male upon evolution. Bulbapedia keeps a heavily researched list of all known XY glitches, and none discovered have involved Pokemon gender changing on evolution.
The only Pokemon who has ever had a gender-change-related glitch is Azurill to Marill. Azurill having a 25% male ratio and Marill being 50% male meant 1/3 of all female Azurill would become male on evolution. However, this glitch only existed in Gen III-V, and was fixed in Gen VI, so it's very unlikely something similar would be happening to your Gen VI Delphox.