I used to experience this nauseous feeling whenever I do pull-ups or deadlifts, but as I get used to doing them more and more often, the feeling goes away. Just remember to breathe, drink small sips of water every now and then, and focus by looking into the mirror or something. Also, for deadlifts, I like to pause at the top and contract my back a bit, taking in a quick breath as I do that, and slowly lower the weight back onto the floor again.