It sounds like she's already had a colonoscopy and an OGD (one down below, one up top). If those were both negative (assuming they took some biopsies), there is no further imaging, camera or blood test that is going to show anything new as they, I'm sure, will have ruled out UC, Crohn's, coeliac...or any other IBD...and at the same time, you would hope they'll have ruled out anything more sinister.

Unfortunately, IBS and food intolerance sucks. In some ways, it's good that it's not UC or Crohn's, because then you're on steroids or worse, and risk having attacks that hospitalise you and may even mean an operation and a stoma in a real emergency.

Have you tried allergy testing? I wouldn't necessarily bother going back to gastroenterology if you've been worked up twice and they've come up with nothing.

I know people who've gone through similar ordeals with undiagnosed food intolerances. My dad, for example, has coeliac...but is also bizarrely intolerant to chillies and some fruits. He also refuses to go and get formal allergy testing and puts it down to "autoimmunity". Another relative is bouncing around specialists, one days has chronic fatigue syndrome, next fibromyalgia, and has had ongoing intolerances for over a decade.

I honestly don't think an ultimatum will help...and you'll risk not empathising with how she is feeling (she's going through shitty and embarrassing symptoms, probably guilty about making your life harder, guilty about wasting money on tests that have shown nothing, and at the moment, it doesn't sound like she's ready to confront it all again), but you're certainly right to be concerned about her health and wellbeing. Burying her head in the sand isn't going to make the problem go away...but I can certainly see with why she doesn't want to go through a bunch of invasive tests yet again, only for them to come up with no answers or solution.

Best of luck to you and your girlfriend. She's very lucky to have someone around her who cares for her as you do, but I think an ultimatum is likely to blow up in your face. You're more likely to get success with a very heartfelt conversation about how worried you are. When she understands how much her wellbeing means to you and that you really are there for her, she might come round for another trial of finding out what's going on.