My Nips!

well just the one, the left one. im 38w today with my first. hes definitely a stubborn one. I have all the signals of labor coming and hes clinching onto my ribs for dear life. last week my nips startes to feel sensitive like in the first trimester. the last few hours my left one is tingly, sensitive, and heavy. I know everyone is going to say its the milk coming in but i have yet to leak or show any signs of it before. why would it only be the one boob?