Hi,
Wow, it's hard to say. Do you have other symptoms? Pregnancy of course could be one of the answers. See what else you are experiencing--dry skin, stress, pain, etc. I know your feeling of unease at not knowing what is going on. Your doctor will want to know more too. Start a journal and really write it down when the nausea comes on, what you were doing before, what foods trigger it, etc. Good luck and keep posting.