The differential diagnosis for proptosis and periorbital swelling is broad and includes infectious, malignant, vascular, and rheumatologic etiologies. In this study, we report a case of carotid-cavernous fistula as the cause of acute-onset unilateral proptosis and periorbital swelling of the right eye in a 44-year-old female patient whose symptoms were initially attributed to possible immunoglobulin G4-related disease (IgG4-RD). The patient initially received antibiotics for presumed cellulitis and steroid treatment for a possible autoimmune cause, however; her autoimmune work-up was negative. Radiologic imaging later confirmed that she had a direct spontaneous carotid-cavernous fistula. She experienced significant improvement in her symptoms and vision after embolization treatment. Due to the risk that a carotid-cavernous fistula will progress quickly and cause neurological damage, this is a key diagnosis that should not be missed in patients with acute-onset periorbital and visual symptoms. Rheumatologists should include this condition in the differential for any patient who presents with periorbital swelling and vision disturbances.