Green stool/diarrhea may be from green vegetables, fruit juices, iron supplements, and maybe from your new drugs. Green
diarrhea may also be from excesive excretion of bile
, because of their reduced absorption in small intestine (due to some intestinall process, or simply from increased gut motility (and hence no time for bile acid absorption).
Well, normal US doesn't rule out gallbladder disease (a dense bile, called biliary sludge, may be easily overlooked). HIDA scan is more accurate.