There can be various causes for nausea and unpleasant feeling you are experiencing which include Chemicals or toxins, Motion sickness
(disorder which cause nausea and vomiting in sensitive individuals while traveling), gastritis, peptic ulcer
, severe hypertension
, unpleasant smell or pictures, brain tumors, morning sickness
, radiation therapy etc.
Based on physical examination, past history and other associated findings, the doctor can be able to confirm whats going on with you.
Hope it helps!