As purplechicken74 says, IBS is of course a common co-morbidity to fibro, and IBS is more gut, but comes with stomach problems too. I pinpoint my stomach problems separately (as hyperacidity), yours might be that if as like me you don't tolerate anything spicy. If you're like me that'd also mean completely avoiding everything sour, too hot or too cold, onion-like (incl. chives, garlic, so almost all spice combinations, dressings, soups etc.), only girding your waist loosely. Does any of that fit too?
My problem at the moment is a delicious variant of yours: All my supps helping sleep & muscles are acidic...
After researching, I've decided to increase psyllium & linseed oil (both gut & stomach) and add the probiotic yeast Saccharomyces boulardii ('Perenterol') (gut).
The details, if anyone's interested:
It's amino acids (mainly GABA) and magnesium forms that are helping my sleep and my muscle stiffness. (Passionflower is I think helping sleep, and at least not harming, not acidic (also herb, not supp), but increasing doesn't improve, have to watch out nothing zombifies me.) Magnesium can cause loose stool or diarrhea, does me. The gut-friendliest forms malate and (bis)glycinate are compounds with malic acid and the amino acid glycine. Due to insomnia again (despite perfect sleep hygiene) I've added and now increased L-glutamine for falling asleep better (Creola17's recommendation above for 'leaky gut', which I haven't got). L-glutamine is - yep - also an amino acid (and also may have been responsible for increased focal seizures I had last time I tried, but GABA and praps the ). I've increased them from 1,5 to about 3g altogether, and would like to try to do so further. What I've experienced is that capsules seem to spread the effect between stomach and gut, at least they are better for me than pills or sublingual intake, which concentrates them more in the stomach.
Yeah, I know, typical med problem too, one med can lead to 3 others, and I don't just tolerate most of them (stomach, gut, skin), cos the side effects I get there are 10x as much... So I had a look this morning for additional supps/herbs with no side effects that I'd forgotten or didn't know, so I can increase the acids further. What I've found is that psyllium, which I take for diarrhea (it's also for constipation if you take it with a lot of liquid) can help heartburn, so might help. Bentonite (clay) and zeolite (mineral absorbent) don't sound too good to me, as I never had any success with healing earth/medicinal earth/sepiolite/Fuller's earth. Also tried probiotics to no avail for months and months, several times. White cabbage salad is sposed to be good. And linseed oil which I need to "drink" anyway for omega 3 (blood fats). Saccharomyces boulardii ('Perenterol') is a probiotic yeast for diarrhea, which I know has worked for me in the past, now I've seen it may also work on the stomach if you get acid problems (heartburn, GERD/reflux) when your stomach trying to compensate for too little acid (cf. pubmed e.g. PMC3296087).