It's happened to me - I think diet may have something to do with it. I don't know about you guys but when I have almost an empty stomach I have a great trip but when I have a full stomach or half full, it will 'clench' sometimes or I will get a slight 'wave' of nausea which usually passes just as quick as it comes on.
It's never enough to stop Me though.
Also, I think that it can be the nerves. When I get really nervous my stomach acts up. I feel like I want to puke.
That happened when My Mom died and when I used to get in a lot of trouble, the fear of getting caught would make me sick to my stomach, so, I think that since DMT can be so overwhelming because of its rapid onset when smoked, it can be kinda compared to getting sick after a roller-coaster ride or a stressful event.
I think the SIZE of the 'Hit' and how efficiently it is vaporized or smoked [ Gently ! Don't kill the Deemsters Maaa-aaa-aan !] can be a factor too ... sometimes the first hit is so concentrated and strong, that trying to take the next one, cross-eyed and fumbling with the lighter, next thing you know the stomach is like " What what what? " and it cringes - You know?
All in All though,I don't usually get sick to my stomach
I'm pretty darn used to the trip [ Elf Space ] nowadays but no matter HOW MUCH I try to prepare for it's rapid ONSET , it can still can be quite overwhelming, like," Whoahhhh-oh-oh, Hang On Hang on Hay-ong Hee-ong Hyong yong uh uh WOW Mannn ! Holy Crap !! "
You know?
That can give 'Butterflies" in the Tummy, too
Make absolutely sure the spice is Completely DRY

before smoking too
I ALWAYS use a dessicant [sp?] just in case and wait until the spice has a 'crunch' to it when I poke it with my finger and mix it around
I want it to feel like dry bone meal, then I knows it's 'good-to-go'.
Everything I say here happened in My own Imagination. The more fantastic it sounds, the more you can count on it being in the realm of Dreams,