Good advice above regarding professional help. Don't fly, or talk to the FAA/AME until you get your situation in order.

In this sort of situation, the FAA is going to want to know the cause and the fix.

If the cause cannot be determined, they are going to want to wait and see if it happens again. I've heard of that being six months to three years.

If the cause was "likely" dehydration, they may still want to wait, not really sure how that would play out but if the records from your ER visit coroborate dehydration (tests indicated that, or you stated that at the time) I imagine that would help your case.