I had an exceedingly unhelpful gastro doctor who basically did a colonoscopy and had me do the FODMAP diet and then shrugged and said if the diet fixed the issue, then why bother figuring anything else out. He also didn't think I needed to see at dietitian to help me out at all. Which I honestly really really did need to see one at the time but I eventually figured it out.

So yeah. I never did any other testing including the upper endoscopy so it's a mystery.