Have you tried the Check a Symptom box on the upper left hand of the page to see what it says about your symptoms. The gastrointestinal system is very complex and only a doctor can diagnose. IBS can cause alot of symptoms including lightheadness in some people. In fact severe gastrointestinal symptoms are caused by illnesses that are very uncomfortable but are not considered dangerous. Then, again, it could be due to something else.
Did the doctor do anything other than a physical examination? Did she/he take any blood work,urine or stool tests, x-rays, ultrasound? These are some of the tests doctors may do when confronted with abdominal pain. If she/he did do them perhaps you can feel better that this really is IBS.
On the other hand its very clear that anxiety causes increased pain, shortened breathing patterns (which can cause dizziness, etc.) If you're feeling particularly anxious you might want to do relaxation/breathing/meditation type exercises and see if your pain abates.
Paste the url below into the address bar at top for more of the symptoms of IBS; mucus in the bowels, it turns out, is one of the more common symptoms, as is - in severe IBS - dizziness. Perhaps your physician needs to more aggressively treat your IBS.
Good luck!
http://www.healthcentral.com/ibd/guide-154495-75.html