Is there any idea of what causes ibs? why it's different in other people. can bacteria overgrowth play a role

Many factors: There are many factors that may contribute, including genetics, imbalances of intestinal flora (including both yeast ; bacterial overgrowth) ; lack of digestive enzymes. Stress usually aggravates it. Many may have undiagnosed parasites (hard to find) or infections like lyme disease. Dietary changes often help. See http://digestive.Niddk.Nih.Gov/ddiseases/pubs/ibs/#cause ; http://abt.Cm/16tmrig.