Yes, that diet drug is very bad on the gut. I think maybe it only makes you lose weight by giving you D. In any case, give your gut another week or so to adjust. You might try an anti-gas pill as Lil suggest. Some Activia or Stoneyfield yogurt wouldn't hurt you any either (unless you can't tolerate dairy); both have goodly amounts of good bacteria in them which is always good to put into the gut after any sort of upset (like stomach flu or food poisioning or colonoscopy prep or anything that gives you D).
Make sure you also aren't consuming any diet candies, gums, mints, sweets, ice creams, etc. Most all reduced-sugar and no-sugar products contain sugar alcohols (you can find them broken out under sugar on the nutrition label, or see them as ingredients ending in "itol", such as sorbitol, mannitol, xylitol, etc.). Sugar alcohols are also known to cause D, gas (usually unbearably stinky), and bowel rumblings. For some people, the reaction to sugar alcohols is worse than in others. 1 gram in a couple of pieces of gum once laid me up in bed for two solid days with cramping, horrible noise and bad D, then I had another month of D and pain after that.