Cause of memory loss after migrain headache

[vacafisch]

My 68  year old mother was experiencing migrains.  During the headache, she had a lightning flash in her vision and was slightly disoriented and didn't know where she was after it took place.  She went to the hospital and they did a cat scan said it wasn't a stroke,but was the migrain or sinus issues.  She had another one that caused her body to feel numb all over and also in her gums and mouth.  She went to the hospital and they said she had a cluster migrain and gave her medication to treat the migrain.  She also had a flu shot at some point during one of the visits.  
She was nauseated and threw up for a few days.  Then 2 - 3 days later, she lost memory and became very despondent and just kept her head down and didn't talk.  We thought she was just sick and in pain from the headaches as they had not gone away.  We then realized she couldn't remember how to do basic functions such as writing her name, cooking, folding clothes remember numbers, add, count etc..she was very childlike and giggled and nothing seemed to bother her.  At this point, we took her back to the hospital and they did extensive neurology tests.  MRI, Cat scan, contrast cat scan, EEG, Pet scan, spinal tap.  Nothing shows up on any of the test.  The second day she was in the hospital after working with a therapist she was able to write her name, the next day she was able to write her name and she did get her memory back though she is a little slow in communicating.  The headaches have continued and she has had the flashing in her vision.  What could be the cause of this?  What can be done to determine why this happened and to prevent it from occuring again. Are there any more tests that we should request?  There has to be a cause and we need to know what suggestions you might have.  Thank you,