The DMT experience can leave you without any sort of awareness of breathing, to the point where it feels 110% like you've stopped breathing or are getting choked up; you're not. Part of the experience. Breathe those last few breaths through your nose as it's ramping up, lay back, and allow whatever happens to happen. You're body knows what to do, just lay back and go. Such a wide variety of effects that DMT can bring up, that you really should expect the unexpected, in all it's forms and/or feelings.

Another thing - don't fight it, get up, flail, anything, just be with the experience, on its own terms. Let it wash over you. Just let it. It's short in effects, sacrifice yourself for those few minutes.