Have you had any blood tests done for food allergies/intolerances? I've had GERD, stomach cramping, bloating, and alternating C and D for years. Only now that I've gone gluten-, dairy-, and corn-free have I found any relief. Food intolerances can cause a whole slew of symptoms, not just GI ones.
You might consider going gluten and dairy free for a couple of weeks, just to see if it helps. This is something you can control and isn't all that difficult to do if you do some research on gfdf recipes and food ingredients ahead of time.
Have you had your thyroid checked? Thyroid problems can also cause a wide range of issues.
Have you had a nutritional panel run, including the B vits, vit D and ferritin? If not, it should probably be done anyway because you're probably not absorbing nutritients from your food due to the IBS. I had also been suffering from severe fatigue, joint pain, headaches, etc. These cleared up since starting supplementation (after testing to determine deficiencies, of course) in combination with the new dietary changes.
There's a group of folks who see IBS as a symptom of something else, rather than a dx in its own right. It really is a catch-all term used by drs anyway when they don't know what the real problem is.
I hope this helps.