This type of thing is extremely common. When you're body is so wired all the time from anxiety, you will even start looking for the symptom, even if it has disappeared. If you have determined that the nausea feeling centers around your mouth, then you must focus your attention there. Give it some friendly attention. Say thank you for the fact it is trying to protect you. Take a couple of minutes to do the breathing and the posture tips I posted earlier. Remember, your symptom does not have to go away immediately, just acknowledge it is there.
When you do this regularly, your nervous system will be less tense and less wired, making you more resilient and reducing the likeliness of such symptoms appearing. Of course, you do have to keep listening to your body and take a bit of time to relax every now and again. Even if it is only 5 minutes.