Cause of one bulging eye?

Thyroid, others: In the 19 year old age group, thyroid malfunction is the most likely diagnosis. Less common are tumors behind the eye, some primary and some metastatic. If the bulging is accompanied by pain, especially on eye motion it could be infectionl this always requires evaluation and you should see a neuro-ophthalmologist or oculoplastic surgeon (not an optometrist).

Bulging eye: The most common cause of unilateral proptosis in an adult is thyroid eye disease.