Dog bit me on my face

My dog, whom I adopted from the street almost two years ago and who has never shown signs of aggression before—except for growling at another dog if it comes near his food—bit me on the nose last night. He sleeps on my pillow, and I often wake up during the night and turn to the other side, which wakes him up or moves him. Last night I did the same, and he growled and snapped at my face three times. I would think he was asleep and not aware of what he was doing, but he did it three times, not just once. He didn’t break the skin, but my nose is red and sore. I’m pregnant and we’re having a baby soon, and this really worries me. Please give me advice and an explanation for why he did this.