IBS (irritable bowel syndrome) is a diagnosis of exclusion. All that means is that every other possible cause for bowel issues has been investigated and ruled out. Whenever you see the word "Syndrome" be aware that it indicates not a known disease, but simply a description of symptoms that provides a convenient label. End result - they don't know what's causing the symptoms.
Since you suffer with constipation, you'll have to try the usual remedies first. Increase fiber, but do it very slowly or you'll feel even worse. Make sure you drink a lot of water during the day, and not just sodas, coffee and tea. If you eat a lot of junk food, you'll need to change your diet to a more healthy one with more fruits and veggies. Meat is a big culprit in constipation as well. Don't cut