Once i was inside a pharmacy and suddenly my vision went all black, then all white. what happened exactly?

Visual Distrurbance: Sudden vision changes can be due to the following: bleaching out of retinal photoreceptors (if exposed to bright light suddenly); blood vessel fluctuations (can be associated with headaches sometimes); medication side effects (check meds to see if vision issues a possible side effect); other optic nerve pathway issues (high eye pressure, brain pressure, masses in brain --very rare). See eyemd.