I believe many things can be misdiagnosed, IBS being one of them. My friend was diagnosed with IBS, but I really believe that she has Celiac disease (aka gluten sensitivity) or some kind of dietary thing.

The reason I say this is not to say, "Oh! None of you have IBS! I'm right, you're wrong! Mwahahaha!" No, not at all. XD I'm just putting it out there to alert you to the fact that you might not have IBS. Ask your doctor for tests if you continue to have problems even when you have a greater hold on your nerves.

http://www.google.com/search?hl=en&q=Misdiagnosed+IBS

http://www.wrongdiagnosis.com/i/irritable_bowel_syndrome/misdiag.htm

http://digestive.niddk.nih.gov/ddiseases/pubs/celiac/ (I actually have this. If you have any questions about it, feel free to message me).