If you go on Zooplus/Bitiba, there are a lot of sensitivity/gastro foods, you might be able to find something to tempt her there, one of my cats with a sensitive tummy was best on James Wellbeloved. The treatment for IBD and intestinal cancer is actually the same, so I'm not sure I'd put her through loads of tests for an answer. I'm not sure why they gave antibiotics though, they can make diarrhea worse. Have you tried any probiotics? An easy one is live natural yoghurt, although you can get special cat ones.