Nausea is a very vague symptom, yet it is a common complaint. By itself, it may not be very serious, but when it is associated with vomiting it requires investigation for its etiology.
Since there are literally at least 100 causes for nausea, I would like to cover only the most common. In a female, of child-bearing age, pregnancy needs to be considered before all other possibilities. If sexually active, if birth control methods are not perfectly followed, and if a menstrual period is missed, think of possible pregnancy.
Irritable bowel syndrome is the most common cause in females and males. Drugs and gastroenteritis would be the next etiologies to consider.
Finally there is a long list of medical problems with which nausea is associated, including: diabetes, gastritis, viral illnesses, renal failure, kidney stones, heart attacks, inner ear problems, brain tumors, etc.
Depending on what other concomitant symptoms one has, evaluation should begin with a history and physical exam, but may require blood work, upper endoscopy, abdominal ultrasound and small bowel x-rays as part of the gastrointestinal work up. If that is all negative, other specialties may need to evaluate the patient for causes of nausea.