More than likely it's her anal glands that need to be expressed. That's a classic sign (the sit down and spin or scoot) that they need to be expressed. I doubt it's the actual food itself but if her stools have been loose from changin her food that could be a reason she's doing it. This is going to sound gross, but needs to be said. If the stools aren't hard enough and if the anal glands are not in the right positions, then when they poop it doesn't press up against the anal gland right and when that happens they don't get expressed. This is a normal function that all dogs do.
Here's an article I prepped after quite a bit of research I did. There's a video of a bully scooting too that you can watch.
Anal Gland Care - Everything You Need to Know
Now if the ears are itching it could be the food or just dirty ears that need to be cleaned.