Kindred by Octavia E. Butler

On her 26th birthday, Dana and her husband are moving into their apartment when she starts to feel dizzy. She falls to her knees, nauseous. Then the world falls away. She finds herself at the edge of a green wood by a vast river. A child is screaming. Wading into the water, she pulls him to safety, only to find herself face to face with a very old looking rifle, in the hands of the boy's father. She's terrified. The next thing she knows she's back in her apartment, soaking wet. It's the most terrifying experience of her life - until it happens again.