I've noticed a weird random overwhelming sense of nausea (feeling like I am going to throw up) occasionally, almost (and sometimes!) to the point of vomiting... but there is no headache or dizziness involved. It does not seem to be a reaction to any medications, either.
It's an uncommon MS symptom, apparently - yay me! MS can effect any part of the nervous system, apparently even the 'vomit centre' of the brain (the result of a lesion) - oh, just great!