In this week's issue of PEOPLE, the Good Girls actress, 31, opens up about her years of excruciating pain that led to a life-changing diagnosis, endometriosis — the reproductive condition in which uterine tissue grows outside of the uterus, causing cramping and chronic pain. Get push notifications with news, features and more. + Follow Following "I remember there was a scene in One Fine Day where there was a lot of running across New York and I got really sick to the point where I couldn't do it," recalls Whitman, who starred alongside George Clooney and Michelle Pfeiffer in the 1996 romantic drama. "I couldn't keep moving. I remember that kind of marked the beginning of a journey of a lifetime of stomach aches for me, and a lifetime of problems for me."