Can geographic tongue flare up because of stress?

Yes but: Geographic tongue does not have any symptoms other than the shapes on the tongue. If you have other symptoms you may have something else going on and i would suggest seeing a dentist specializing in oral medicine.

Oral Medicine: While oral medicine is not a recognized specialty in dentistry, your tongue should be examined to rule out anything other than the totally benign phenomenon of geographic tongue (benign migratory glossitis). Have your oral soft tissues, all of them, not just your tongue, examined by your dentist. If he notes anything requiring further study he will refer you to the appropriate practitioner.

Geographic tongue: is a benign condition affecting the surface of your tongue. Although geographic tongue may look bad, it is harmless. In some cases patient may experience discomfort or burning sensation related to a certain foods. It is idiopathic (not known) permanent condition and no treatment is available. Stress could be a factor, but not a cause.