I wonder if you have ever tried taking red meat out of your diet. Like you, I was convinced I could tolerate it. I had been eating red meat for 45 years, which is how long I believe I've had IBS. Certainly I knew I couldn't have fatty red meat, but I was convinced I was fine with lean red meat.
I didn't realize I shouldn't be having red meat until I discovered Heather's website last spring. My reaction to Heather's advice was that perhaps red meat was a problem for some IBS people -- but not for me! (I also had the same reaction to her advice not to eat dark poultry meat.)
However, out of curiousity to see what would happen, I gradually reduced the amount of red meat in my diet. (It was definitely a struggle to do so!)
I have now been red meat-free for about six weeks. And guess what! To my surprise, I feel so much better! I find I can't touch red meat at all now because I get D and pain when I try.
I now think that when I used to get IBS attacks and I attributed those attacks to other trigger foods, the red meat was, in fact, contributing to those attacks. I also think the reason I thought I could tolerate red meat is because I had gotten so used to feeling rotten most of the time that I wasn't even aware most of the time that I was feeling rotten! But now I'm acutely aware of how I feel because I feel so good these days (except for my acid reflux which I'm working to control)!
If you haven't already, please consider taking red meat out of your diet for awhile as an experiment. You might be as surprised as me at the results!