Hello! Thank you for the query. Nausea can be caused by pregnancy, stomach issues (like peptic ulcer, stomach inflammation, acid reflux), medicines (contraceptives, antibiotics), increased brain fluid pressure (intracranial pressure), stress, anxiety, depression. Stomach cancer is hardly possible at your age, but should be ruled out. I suggest you to start with pregnancy test and some blood work. Next should be gastroscopy especially if you have any upper middle abdominal pain or chest burning. Verify medicines you use (see leaflets for side effects). You should also consult neurologist and psychiatrist (as a last step). Hope this will help. Regards.