I get the same thing! Unfortunately, if it's what I think it is, its anxiety nausea, so beyond coping methods (like deep breathing, which you've already tried) there's not a big lot you can really do
Sometimes I get it before I even eat in the morning of an exam, so maybe you do too? I find it's always a good idea to have something small - hunger will affect your mood and make you end up even worse, so even a slice of toast helps! Too much though, and you risk also making yourself feel worse (because you feel particularly full, you might be more aware of it? Not sure how it works, just what I've noticed).
Also, try cutting down on caffeine! As boring as I sound saying it, I find it helps me somewhat, so it might help you. Don't go cold turkey either if you're used to a morning coffee, you'll end up with caffeine withdrawals, which are arguably worse.
Again, not a big lot else you can do at this point in the year, but if you're at GCSE/AS level, try seeing a doctor about it. It could take a while, but you might get some support from it for next year. Couldn't hurt to try?
Just know you're not alone with exam nausea, and good luck!