Hi and welcome to the forum! I am glad that I can help you.
You want to know the cause of the symptoms that you experience.
Nausea is a symptom that usually accompanies intestinal infections, stress, migraines, eating disorders, etc. In case of intestinal infection nause would be accompanied by other symptoms such as increased body temperature, vomiting and abdominal pain. Collection of air in the stomach may imitate symptoms similar to nausea, until the air is released. During stress most of the body systems (including the cardiovascular and the gastrointestinal system) work faster, so sometimes nausea may be experienced as a response to the stress.
According to the data that you described, you may or may not be experiencing any of the medical conditions listed above. Consume vitamin B6 to help prevent psychological stress. Avoid consumption of coffee or tea because these drinks contain substances that increase the heart rate and may cause nausea in rare situations. Avoid tea or substances that help in reduction of body weight because they may cause nausea.
Please keep in mind that I provide medical information only. I am not able to diagnose medical conditions online. Please contact your doctor for further advice and information about diagnosis and treatment options.