I had a similar reaction to Prilosec OTC. A friend of mine told me that it had even caused damage to her stomach and intestines when she was taking it as much as I was. Then, another friend of mine told me about the connection between GERD/IBS and food sensitivities.
After discovering what my food sensitivities are through an elimination diet, I no longer take medication for my GERD, and I have GERD pretty bad. It turned out that the reflux was definitely caused by food sensitivities in my case and is probably the case for many other people, too. Improving the diet helped with the IBS, too.
I can relate, because I have no doubt that I had IBS my whole life even though I wasn't diagnosed 'til I was a young woman.
For occasional breakthrough reflux, taking Benadryl actually helps, because these food sensitivities are actually an immune response that just isn't as severe as what is termed as "true" allergies. Natural medicine refers to food sensitivities as "hidden food allergies", because food sensitivities will still make you sick. Food sensitivities won't kill you immediately with anaphylaxis, like true allergies will, but they'll still make you sick. They're still not good for your body, even if the food is the healthiest food on the planet. And, since I have environmental allergies, Benadryl makes sense for me.
This is why I steer clear of psyllium fiber for my IBS. My GERD gets aggravated by psyllium fiber. My naturopath told me that psyllium is rather irritating for a lot of people, too. This is why I mention this so often on this website. Flaxseed meal or other fiber supplements that don't irritate your GERD are better for people sensitive to psyllium fiber.
Be sure to talk to your doctor about the reaction you're having to the Prevacid. Your doctor may wish to prescribe something else for you that won't make you so nauseous. Regardless of whether you take this medication or something else or choose to not take the medication, you really should tell your doctor what kind of reaction you're having to the Prevacid. I've been there, but mine was a case of GERD caused by food sensitivities, so be sure you talk to your doctor before doing something else.
Oh also i was diagnosed with IBS at a very young age.