Here is a list of drugs, some of which can cause loss of consciousness: http://onestudent.org/wp-content/uploads/2010/09/common-drugs.pdf
The loss of memory happening so fast doesn’t make sense.
How do you know the speed at which a memory loss takes place?
Is it known whether she had anything in her system before they picked her up? There may have been circumstances that weren't admissable that aren't publicly known. She may already have been under the influence (of her own accord or otherwise). Or she could have been on medication that would have caused something else to act more potently than it might have done otherwise.
Alcohol and some drugs can cause Anterograde amnesia:
Anterograde amnesia can also be caused by alcohol intoxication, a phenomenon commonly known as a blackout. Studies show rapid rises in blood alcohol concentration over a short period of time severely impair or in some cases completely block the brain's ability to transfer short-term memories created during the period of intoxication to long-term memory for storage and later retrieval. Such rapid rises are caused by drinking large amounts of alcohol in short periods of time, especially on an empty stomach, as the dilution of alcohol by food slows the absorption of alcohol.