I had my Nissen last October. I was diagnosed with IBS a couple weeks later. My IBS had shown itself in the months previously though. In the months preceeding my surgery, I was on every PPI on the market. Within one day of starting the Nexium, I started in with the big "D". I still have some major intermittent nausea which I attribute to the Nissen, but the rest is just the IBS. It can be frustrating at times dealing with the symptoms.
My GI doc still has me on a double dose PPI and following my anti-reflux diet and lifestyle. I was doing it pre-surgery and he wouldn't give me permission to change after because he said my IBS symptoms would probably get worse. I think maybe my diet is keeping the IBS from getting worse.