What size dose are you smoking? Larger doses can tend to cause nausea where you may be able to achieve a breakthrough by backing off on dose so you can fully let go.
What device (bong, bowl, GVG, machine....) are you using and with what kind of lighter? Depending on which one you use and how you use it, you may be more or less likely to burn the DMT. Nausea can is a common symptom of burnt DMT.
You may also wanna consider not eating so much, or eating light before the experience. Go into the experience on an empty stomach and see if the nausea is still there.
"Science without religion is lame. Religion without science is blind" - Albert Einstein
"The Mighty One appears, the horizon shines. Atum appears on the smell of his censing, the Sunshine- god has risen in the sky, the Mansion of the pyramidion is in joy and all its inmates are assembled, a voice calls out within the shrine, shouting reverberates around the Netherworld." - Egyptian Book of the Dead
"Man fears time, but time fears the Pyramids" - 9th century Arab proverb