Had nauseia for over two weeks. went to dr he said it was hard to diagnose since it's my only symptom. what can be causing this?

Difficult to say: I would need to take a complete history and perform a focused physical exam. Have you been vomiting, too? Just at a glance, two possible etiologies that come to mind in a young woman with recurrent nausea +/- vomiting for two weeks include medication intolerance (antibiotics) or early pregnancy. Other possibilities include endocrine disease, drug/alcohol abuse, exposure to toxins/poisons.