Only a doctor can diagnose you with IBS and its based on a "specific cluster of symptoms" as there is no specific test for it, but there are specific guidelines for diagnosing IBS. Although it sounds like it and IBS is common.
You might want to look over this site.
International Foundation for Functional Gastrointestinal Disorders
Let us know how the colonoscopy turns out or if we can help in anyway. The colonoscopy can rule out things and make sure there is nothing really serious.
If you have any questions ask away.