The vomit sensation you are talking about is known as gag reflex. It is a protective mechanism of the human body to avoid the entrance of any unwanted material down the throat. In your case, you are experiencing the reflex while brushing, that means your brushing pattern might be wrong. You are taking your brush to much deeper areas than required. Also, you might not be cleaning your tongue properly. Visit your dentist and ask him about some correct brushing patterns and start using a tongue cleaner.
Whenever you experience gag try to focus on some other activity or object, such as your big toe. It diverts you and thus the gag stops. Avoid mouth breathing, that increases the gag. If the problem persists even after changing your brushing/cleaning pattern and avoiding mouth breathing there is more to it. Pay a visit to both your dentist and gasteroenterologist.
Suggestions offered by doctors on Lybrate are of advisory nature i.e., for educational and informational purposes only. Content posted on, created for, or compiled by Lybrate is not intended or designed to replace your doctor's independent judgment about any symptom, condition, or the appropriateness or risks of a procedure or treatment for a given person.