Yes, AIHA can come on that quick. My puppy (not this one) two years ago,was playing and happy. Then the day before Christmas she was just not herself, was acting lethargic, vomited, did not want to eat, had a 104 temp. I took her into vets on Christmas morning and she was diagnosed with AIHA.. Her gums were almost white, they had been fine the day before.
Its auto immune, the dogs body starts destroying its own red blood cells. They get anemic really fast.
I am sure that its not that, but always worry when someone writes in and says" my dog is not acting right and is very lethargic and sick acting.