I have to admit I have a bias in writing about "random," unexplained nausea. I had a mesenteric blood clot, myself. It caused severe pain, but I also remember thinking I never wanted to eat again. This went on for about a week, until blood thinners broke up the clot.
I also have some experience with nausea after transient ischemic attack, also known as mini-stroke. There was no reason I should have nausea that just wouldn't go away. I was NPO (nothing by mouth) in my hospital room. It couldn't have been anything I ate, because I hadn't eaten anything. However, I had really awful nausera with nothing on my stomach.
Vomiting is the actual upflow of the contents of your stomach, and retching, or dry heaves, is intense motion of the muscles lining your stomach without any stomach contents coming up through your mouth. Nausea is the feeling that vomiting or retching are about to occur, whether or not they occur.
Sometimes nausea is a sign that you are dehydrated or your electrolytes are seriously out of balance. In the latter case, it isn't enough just to drink water. You have to drink water with electrolytes, even if they are a little salt or a little sugar (about 5 grams, or a teaspoon, of each, in a quartt/liter of water).
Sometimes nausea is your gut's way of telling you that eating or drinking would be harmful. It may need "rest" while you have an intestinal infection, or in the examples in this thread and my own personal experience, a blood clot stopping flow to the intestines.
Sometimes nausea is the result of injury or inflammation to the brain, or the use of drugs to alter consciousness. It is not unusual to be nauseous after anesthesia. This is simply your body's way of telling you it is not a good idea to eat or drink when you may lose consciousness and choke.
Nausea is very common during the first trimester of pregnancy. The embryo is very sensitive to toxic chemicals, and the less the mother eats, the lower the risk of teratogenic damage that can cause birth defects. As the embryo becomes a fetus, usually (but not always) the mother's morning sickness improves.
When nausea is "random," however, there are other things to consider.
One is post-traumatic stress, most often after an event involving food. It's not just that if you are struck by a meteor while eating a bacon avocado cheeseburger with chili fries, you will experience meteor anxiety the next time you eat a bacon avocado cheeseburger with chili fries. Basic food scents, especially vanilla, imprint on the brain. There are tiny amounts of vanilla in baby formula. Vanilla becomes associated with being held by mommy. Forty or fifty years later when mother dies, the tiny trace of vanilla that is also added to ketchup can trigger a sense of sadness when eating French fries. Sometimes there are subtle associations of food odors with sad or traumatic events that cause nausea, even though they are not on a conscious level.
Another cause of "random" nausea is chronic dehydration. This is most likely to happen with people who make and follow resolutions for bettert health. If you aren't getting enough fluid because you have scaled back fluid consumption to match lower food consumption, or if you don't rehydrate when you work out, nausea can be an ongoing but not constant problem.
When nausea is caused by emotional associations, it tends to go away with time. If it's too much to keep on dealing with, see a doctor and be sure to mention surgeries, chemotherapy, traumatic events that occurred just before the nausea became chronic, and any drugs you use.
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