I don't have IBS (I do have separate digestive issues, a hiatal hernia and gall bladder issues both diagnosed way before the Fibro).
I described my exhaustion to the doctor as if I've got new born triplets, two have colic, none sleep and no one helps me lol. The key marker was that I was just as exhausted even if I'd had good sleep. The pain I get in my muscles and joints and just under my skin feels like it does when you have bad flu and I get dermatome pain on my back and arm. I'm in low level pain 24hrs a day but manage it until I have a flare. Sleep is difficult but my meds help with that now.
Bloods ruled out thyroid, menopause and all the usual culprits. Investigations for arthritis also negative. At one point Dr though I might have MS but that was ruled out by MRI.
Finally got diagnosed with Chronic Fatigue Syndrome AND Fibro, all the investigations took about a year. Most of which I wavered between "am I faking/just moaning?" to "do I have cancer?"
It's been a gamechanger. I'm a Uni student and the Fibro Fog means studying is tough. I have to use a mobility scooter most of the time now but I'm not faking, and I don't have cancer.
Pacing really helps, heat too (I have an electric underblanket in bed and for the sofa and I've been known to wrap it around my legs when I'm studying). I've just started to go to Aquasize (but go at my own pace) and swimming/steam room 3x a week, even for only a few laps.
Mostly, it's about recognising my body or brain limits and respecting them.