Sudden weight loss can cause menstration problems. Have you lost weight from being sick with your IBS? If you've ever been irregular before, this may just be a new manifestation of the old problem. Back when I had periods, they were never regular and they'd take spells where sometimes they were more regular than other times and sometimes I'd skip periods for months. So if that's happened to you before, I wouldn't be concerned.
If you are usually clockwork regular, then yes, you may want to go see your gyno and get some hormone tests. I don't know your age, but some women can start menopause as early as their late 30's. Hotflashes and irritability, etc. would give you an indication if this is happening to you. A check of your hormone levels can also confirm that. Oh, and hair loss can be a sign of hormone imbalance (although I have lost hair without having a hormone imablance, so it's not definitive).
Unfortunately, your IBS may be masking some more serious female issues, like polycystic ovaries or endomitriosis. Both cause hurting in the same area as IBS and periods and hormone fluctuations of other sorts can cause gas and diarrhea or constipation. One of our girls posted a couple of weeks ago a news article about ovarian cancer causing the same symptoms as IBS. If they didn't give you blood work and a full look-over when they diagnosed your IBS, then you should definitely go to the gyno and make him or her check you out. Out of everything I've listed, the cancer is the least likely to happen, so don't get scared and feel doomed; just see your doctor and get sorted out if you don't see a more obvious explanation.