I'd like to make a suggestion for something else you should check. Ask your doctor to check you for celiac disease. It is an autoimmune disease that is induced by wheat gluten. The initial test is for antibodies to gluten (IgG and IgA) and IgA antibodies to a self protein called transglutaminase. If you have this disease and you eat gluten (breads and actually many products) your immune system mounts an attack on the gluten along with attacking your small intestines. It can cause atrophy of the villi in your intestines that absorb food. Since you aren't absorbing food properly all sorts of symptoms can occur. Diarrhea is often a symptom but so is constipation. You can get yeast overgrowths because of the imbalance of nutrients and the presence of immune cells in your gut. The gut is damaged and leaky causing more irritation and symptoms.
This disease used to be thought of as a rare childhood wasting disease but it is now becoming clear that many of the nonspecific intestinal disorders like IBS are really celiac. People suffer for years without a diagnosis because many doctors are taught in medical school that it is rare. It cause all sorts of problems because of the malabsorbtion of nutrients needed for good health.
Some people get really skinny on this disease but others (I think like me) keep our weight on via a mechanism called metabolic resistance. We have genes that shut down our metabolism rather than giving up the fat.
If you test positive on the blood tests they will want to do a intestinal biopsy. This biopsy goes in through your mouth and stomach to your small intestines. You can't see the damage with a colonoscopy (probably what you had for IBS diagnosis?).
You need to be eating gluten to take the blood test because the antibodies go away if you are completely off gluten for long enough.
You also might want to ask for an antibody test against candida proteins. In addition to celiac I also had really high titers of antibodies to yeast.
I dont' know what you are taking antibiotics for but try to get off of them as soon as possible. They really aren't good for you long term.
Give this some thought and don't let your doctor dismiss your questions unanswered about this. It takes some people many many years of ill health to finally get a diagnosis that lets them get well. If you want to read more take a look at the celiac support forum
http://forums.delphiforums.com/celiac/start