No matter what food I give my girl, she's always got the foulest smelling arse afterwards. Any ideas on why?

Have you had any fecal tests done? We are dealing with a bout of giardia in our latest rescue dog and bad gas was one of the symptoms. And by bad I mean, they make a person's eyes burn.

How often have you been changing her food?
What foods have you tried?

The only experience that I had with Okami having bad farts was when I fed her the Blue Buffalo Wilderness Salmon Treats... I went out and bought a different brand Salmon treat, and found that it is just salmon not the Brands itself. My guess is there is a possible food your dog is having a bad gassy reaction to. Have you changed anything to her diet as of recent? New treats, new food, etcetera?

Also if you change the food too often you can upset the stomach and get bad gas, runny excrement.
Check with your vet to have them run a food allergy test, and bring with you everything you have been feeding your dog since this started. If you don't have the bags, look up the ingredients to every thing and print them off/write them down. It will make it easier for your vet too to know what allergies to test.

If they do in fact find something wrong, the solution would be simple at that point, find a food and/or treat that doesn't contain the food issue in the ingredients list.

Also food for thought, when switching foods the typical safe ways of transition are as follows:
Take the old food and slowly add the new food over the course of 14 days until you are feeding nothing but the new food.

Or another really great way is feeding a bland diet and slowly transition the new food in.
As far as Bland diets go, my personal favorite is feeding boiled beef, another is boiled chicken, or rice.

You may notice gas issues or runny poop still, but don't falter in your schedule. Once you pick the right food, the problems should go away soon.

And if your vet recommends any food that you can only buy at the vet, don't. Usually they are only pushing it because of sales and sponsors, and the food is usually not very good for them to say the least. There are plenty of great threads right here that give plenty of advice and breakdowns of brands and flavors.

She's not had any tests done lately, but the ones before came back clean. She was put straight on adult food when she was younger so we tried 4 different ones until we found the one she likes, and doesn't give her the runs. Then we moved and found something similar where we are now. We have also got them new treats, but the smell hasn't really changed with what we have given her (except when we give her chicken flavoured food). Thanks for the help