So hard to say. It could be anything from a parasite to an allergy. And some dogs just have nervous stomach. Have you tried adding some fiber to her diet? Green beans, carrots, pumpkin, celery, etc. (No broccoli - gives them gas.)
When you take her to the Vet, what sorts of tests do they run? It seems like a chronic problem like this would have warranted some more in-depth diagnostic attempts.
Wow, that seems a thorough job, in my mind. I do know that there are a few owners on here that have corgis with sensitive tummies, so hopefully one of them will be able to chime in with some good advice. Sorry that I can't be of more help. :(
Poor Cora(love the name) I would still try a different food. My Bella was throwing up for a long time and had sooo many meds. the Dr. said to switch protein sources(we feed grain free) and she's been fine ever since. Not that this is the same but would it hurt??? Just do it very slowely if you try it. Maybe the food is too rich for her?
I would still think it may be the food..try (very gradually) a grain free, different meat source, and if all else fails go with Science Diet i/d which is a bland diet. i/d has made all the difference to our dobe with the weak stomach.
Brown cow all natural cream top yogurt. A teaspoon in her food will clear up her system!!!!! I've been doing this for years and it clears up any stomach problems that arise. At this point I put a teaspoon in Lolly's food for breakfast and dinner. She loves it and it's healthy. Try for a couple of days and you will be suprised.
a couple of things that might help.... bananas and applesauce combined = kaopectate. plain yogurt with live cultures may help replace the good bacteria in her gut that may have been killed off with the antibiotics (especially if she's been on more than one course of meds). The Bananas and applesauce are part of the BRAT diet used for kids the other two things are rice and toast which are easy for kids to digest. I know my sister would used the rice with chicken / broth and the B&A for her puppy with similar issues and it worked great.