I went to see a very nice gastroenterologist last week, who took a thorough history, listened carefully, asked relevant questions and treated me like an intelligent adult. So hooray!
When I told him about the B12 and ferritin levels being low back in the summer, and that I've had IBS for many years, he questioned whether my pancreas is perhaps not producing enough enzymes for digestion, which may have led to the IBS symptoms, and a deficiency in several vitamin / mineral levels. I have non-invasive tests this week to check how my pancreas is functioning. He said that pancreatic insufficiency is hugely underdiagnosed and everyone with my symptoms should be screened just to make sure. He also wants me to see a dietician, just to check my (very good) eating habits.
I'm not sure if this is a possible answer to my symptoms, but I wanted to share with you all that he was concerned and helpful and not the slightest dismissive. I know a lot of people have had some challenging experiences with doctors, and my own GP has been less than fabulous, but this guy was great.