What causes gastrointestinal symptoms with antibiotics for urinary disorders?

GI Symptoms with UTI: Any antibiotics taken for infections, does disturb/ alter gastro intestinal flora. Strong antibiotics does cause gastric upset. By their action on intestinal flora, they cause diarrhea, as well. Diarrhea, if associated with fever & nausea/ vomittings, that is a sign of claustridium defficile infection. A very serious situation, needs attention immediately.