Hello! Thank you for the query. IBS can be diagnosed only if there is no significant reason of gastrointestinal issues. So you can not have diverticulitis and IBS. You have diverticles in your large intestine and maybe this is causing all your problems. Diarrhea, constipation and left abdominal pain are very characteristic for an inflammation of diverticles. If this situation keeps recurring, a part of large intestine should be removed. It is because diverticle inflammation sooner or later will cause intestine perforation. Other possible reason of your symptoms can be small intestine partial obstruction (due to adhesions or inflammation), chronic bacterial infection. This two conditions should be also ruled out. I suggest you to have some more tests. Stool tests and stool culture test should be done at first. Abdominal CT with oral contrast (enteroclysis) seems also advisable. Hope this will help. Regards.