Can nausea be primary symptom of IBS?

Personally, I'd keep looking for an answer.

I agree with you that avoiding the symptoms without finding the cause seems foolish. However, if you can find a way to stop feeling so miserable WHILE you are looking for the root cause, why not do it? It's so difficult to think clearly and have the energy TO look for the answers when you are constantly nauseated and miserable.

Just because it MIGHT buy you some nausea-free time and peace to find the answers, you might want to consider an elimination diet. I'll say why below.

Re: food allergies or intolerances. They CAN start between one day and the next. There are many recorded cases of someone eating something like peanuts all their lives and going into anaphylactic shock one day as they eat peanuts. Doctors understand how an allergic reaction works, but they don't yet know why the body will suddenly start reacting to a food or substance. As they don't know the underlying cause, they can't fix that yet.

Intolerances, doctors seem to know even less. Even if it's proven to exist, doctors may not know the root cause, or the mechanism that's causing the reaction.

That said, some other ideas for underlying causes, and also reasons you could 'suddenly' start reacting that would actually have little to do with a change in you and everything to do with your environment. Although again, these will likely involve diet changes if you are considering looking at them.

1. Leaky gut syndrome - a condition which makes you more prone to developing intolerances and allergies. Although in my experience it involves more severe reactions, so this would be low on the list.

2. Stomach flu - If you have something that damaged your villi, which can even include some stomach flus, that can cause lactose intolerance. My daughter is a celiac, with damaged villi, and her GI doctor has mentioned that her villi have all healed, but that it's possible that the ability of these villi to make the enzymes they are supposed to may not have come back yet. It's been 2 years for her. Certain nastier flus may be able to do something similar, in what I've read on the subject.

3. Changes in ingredient lists - some substances, like the natural dye annatto, artificial dyes, or gums like xanthan gum, can cause nausea and gut issues. A percentage of the population doesn't digest these well and companies change ingredients all the time. All it would take is for one of your commonly used brands to change an ingredient to something your body has trouble with and suddenly, you'd start feeling nauseated all the time. Companies don't announce ingredient changes, so you'd never know aside from the nausea. And most of us don't record the day we ate a new box of product X, so you likely wouldn't remember what changed in your food that day, yes?

3. Changes in your water - City water supplies can sometimes change what sterilization techniques they are using or what chemicals they are using. For some of us unlucky ones, the chemicals they are using don't agree with us and can make us sick. Also, a change in filtration ingredients, if you have any home filters for your water, can also cause some people to have gut issues.

4. GMO's This is going to be totally anecdotal, I'm afraid. My GI doctor has started recommending all his patients drop GMO's from his diet. The reason, as I understand it, is because he's been starting to see stomach and gut issues in his patients that are, well, weird. Inflammation and thickening of the stomach walls, for example. When he researched, he was finding these same issues cropping up in animal studies for GMO research. When he recommended these patients drop GMO'S in their diet, their stomachs started to recover. This is totally second hand information, and only this one doctor's recommendation rather than a fully researched one, but considering that dropping GMO's is a non-invasive way to check this for yourself, it certainly can't hurt, I wouldn't think.

There are other possibilities, too, obviously. I started reacting to certain foods between one day and the next. I, too, can remember the exact day. Doctors were unable to help and I was nauseated and in pain every day. An elimination diet was how I was able to feel better, and after the diet, knowing what I reacted to, I was able to piece together what the problem was and ask for the tests for it. One of my main issues was an enzymatic deficiency; I never would have discovered this if not for the elimination diet.

Wishing you luck, from someone whose been there.