I related that feelings with my past at university,when i was all the day somoking weed and hashish.
I remember i passed trought a phase when suddenly i had panic atacks,i woke up in the night suddenly and afraid thinking my hearth could stop beating....panic of being in small close spaces like trains,classrooms..etc...
It was not only related to joints,im sure they only brought to the surface an inner probñlem here,cz i dissapeared with time while i continued smoking like a mad
Maybe it helps a little,but your symptoms remind me my past experiences very much...specally when you say that its focused on stomach area...i think i felt that feelings of panic there also,and yes,it was simmilar than before jumping from i plane...that incertainty feeling simmilar than panic maybe, and adrennaline
(Lot of energy activated today while writing to you carson,thanks)