Talk to Gastroenterologist on Irritation Bowl Syndrome

I want to know why IBS disease occur i m suffering from 2 years 300tests i hve been done Blood serum stool examination stool culture stool calprotectin similarly urine exam culture etc all medical tests which might b other patient didn't done what i have. My question is what r the causes if IBS is this any bacteria. bcoz after 2 years i finaly met a gastroenterologist he told me 1st will do EGD+DDB i was also a patient of nausea vomiting white tongue he gave me 1st medicine for 7 days and some tests like TSH, ANTI TTG, CALPROTECTIN LEVEL 98.1 when i visit again that gastroenterologist he told me EGD+DDB is not necessary i said ok he finaly gave me medicine for IBS. Alhamdulillah i m feeling much better after 16days.My 2 questions why he didn't EGD+DDB bcoz after 7days of his medication i never vomit and never feel nausea and loose stools r now getting cover. IBS is bacteria or what?