what I have noticed is that if you almost go out of body, but not quite, everything in the physical world seems heavy and labored, including breathing. I think this the fact that your spirit wants to go flying, but is still slightly attached to your physical body. The best way to fix this, depending on your mood, is to take a smaller hit, so you don't feel so foreign in your own body, or take a bigger hit and fully leave your body. If you go big, you won't be in a position to think about your breathing anymore. This is what the full out of body experience is all about, complete separation from your physical dimension. The first time I did a monster rip of DMT, I took a weird gasping breath before leaving myself completely, flying through the most beautiful wormhole thingy. While I was flying, I wasn't a human that needed to breath, just a spirit. Don't worry, I've felt this too. You will be fine.