Explosive diarrhea, gerd and hives

Our daughter has been experiencing episodes of explosive, watery diarrhea, persistent gerd and occasional hives. She was prescribed an antibiotic, and has done an elimination diet, but these efforts have not helped. She is also on bupropion, and I am wondering if this could be symptoms of serotonin syndrome? She takes Tums which helps only minimally. We’re baffled! Anyone’s help would be much appreciated.