So I wanted to throw this out there that my symptoms started the same way. To this day (over 11 years) I have never had any abdominal pain, ever! I did have the other things that you had, plus some great vomiting episodes that frankly scared the crap out of my family and coworkers. This was more about a gallbladder cyst than IBS though. But, I have had all that and I did have IBS. Yes, I said DID because I now have very little of that plumbing left so the IBS is gone! LOL
I am glad you are not celiac, but I agree that it would be really good to try Gluten Free. My wife and daughter have been GF for about 4 years plus FODMAP for my wife and they are doing well. If you need any suggestions on GF products please ask as I believe I have tried them all.
I have to think it is a combination of things though. I hate to tell you that I didn't get a UC diagnosis until the third Colonoscopy. It might just be too early to see the damage. Please keep checking. If it is something intestinal you want it caught early so drug therapies or biologics can be tried.
Please keep us updated and I will help you as much as I can. I wish you great luck!!