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.