The pain in the butt could be impacted anal glands.
Vets recommend high fiber diets for impacted glands.
I don't buy it.

If you don't know how to express the glands to give her relief you should go to a vet to get it done. Don't do it yourself if you have no clue.

My opinion is that impacted anal glands are due to parasites. I would suggest you get Panacur for cats and give her that before you do anything else.
Don't feed your cat dry food if you can avoid it. Not enough moisture and cats aren't big fans of drinking lots of water.

I've spent a lot of time trying to get my cats to eat a home made food. I just get dirty looks. There are a number of really good recipes on the web. The other problem is once a cat finds a food they like they just will not change. My cat is hooked on what I had on hand when she showed up. Full of wheat gluten ech!