Speaking to Grantland's Bill Simmons, Girl's actress Lena said she accidently stumbled upon a picture of Jack and Scarlett while reading an online blog that posts pictures of famous couples who have broken up.
"The craziest thing that ever happened to me, was like, Old Loves is my passion, I check it like once a week," said 27-year-old Lena. "And I was going through it and I saw my boyfriend and his girlfriend from high school Scarlett Johannson."
"She's beautiful, he's beautiful," added Lena. "But it was just so surreal to be looking through this blog that gives me so much pleasure and then there's my boyfriend."
"'I'm not jealous," she said. "It's very sweet. I love looking at pictures of them but I was like, 'My mind is going to explode'.
"I really did think I was hallucinating. It would be like seeing my parents."
Jack, 29, dated Don Jon actress Scarlett for a year back in 2001 and the two were pictured together numerous times, including at New York Fashion Week.
Lena, who has won two Golden Globe Awards for her hit comedy Girls, has been dating Jack since 2012.