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The onset of IBS has been known to happen after a round of antibiotics, a bout of food poisoning or after surgery. It is quite possible that your gut flora was imbalanced because of the antibiotics. Funny because I have yet to come across a doctor that prescribes probiotics along with the antibiotics. They should go hand in hand.
Anyway, your next step is probably to try and get the "good bacteria" back in your intestines. Get some sort of probiotic or eat tons of yogurt (Dannon Activia is fabulous!). Lots of people take, and recommend, Acidophilus. I took it and it gave me major gas. All probiotics will give you gas for the 1st week or 2, but Acidophilus was really bad for me. Instead I take Kyo-Dophilus, Culturelle, I used to take Digestive Advantage and probiotic pearls (enteric coated to
open only in the intestines and not the stomach). All are great. You just have to find one that you like best. Don't worry about
mixing them either, I take a 2 types of probiotics and eat lots of Activia. The way I see it is, the more good bacteria, the better.
If a flora imbalance is indeed the issue, after at least 3 months of continued use of probiotics, your system should be back to normal (whatever is "normal" for you anyway).