What causes severe, acute short term memory loss in a young adult?

Drugs and alcohol are by far the most common causes. A "silent" seizure disorder also might be possible. See a neurologist if drugs or alcohol are not pertinent to your situation. Good luck.

There is a condition called TRANSIENT GLOBAL AMNESIA which can cause amnesia (memory loss) lasting NO more than 24 hours, usually much shorter, just a couple hours with gradual return of memory. This condition most often affects people of middle or older age but it can happen at younger age. This condition is not serious but it can still be a frightening experience.