My doc told me when I started it that with carbohydrates, "If you overdo you'll go pooh!" and he was right! The glucophage does something to the undigested carbs in your GI tract and moves them right along before you can absorb the starches.
Check your carb intake. I have IBS also and I have to keep my very starchy carbs to a minimum. Pastas and potatoes as well as corn meal are real probems for me. I have better luck with bread because it's easier to count bread. A slice of bread is a slice of bread. Pasta is too easy for me to indulge myself and have too much. I frequently have rice as a condiment (2 heaping tablespoons) on a pile of chinese vegetables and meat, use tortillas as wraps instead of a potato or pasta... Substitute a salad for the pasta and put my meat on the salad. The salads give me problems, too, but not the bloaty gassy problems.
Also, some peeps do get used to the glucophage after a while and the diarrhea subsides a bit. A food journal should help you find out where the problem is coming from so you can adjust your diet. Hope this helps.
"People are like stained glass windows: They sparkle and shine when the sun's out, but when the darkness sets in, their true beauty is revealed only if there is light within."
- Elizabeth Kubler-Ross