the symptoms are definitely 'head' based and the gastric element seems to be a consequence of the extreme nausea which causes vomiting. Although I occassionally get a pre-warning by fleeting feelings of nausea a few hours or days before, generally It catches me unawares - I go to sleep and wake an hour or so later very ill.
Could you have Celiac Disease? Some of your symptoms match. www.celiac.org
I hope you feel better.