"I think he has the same kind of issues that I would have if we were hanging out with his ex-girlfriend from long ago," Seyfried told The Mirror. "I think it's always like, 'I'm so in love with this person, so that person must be so in love with her too.'"
Seyfried and Cooper dated for three years after filming the 2008 musical. Last year, she tied the knot with Sadoski just before the couple welcomed their first child, a daughter.
"I'd guess he assumed Dominic is in love with me the way he's in love with me. And it's just not the case, it's sweet," she said. "I'd rather he be jealous than completely fine with it."
Seyfried reprises the role of Sophie Sheridan in Mamma Mia! Here We Go Again, out July 20 -- almost exactly 10 years after the original. In the film, her character is married to Cooper's.
"You do see her. She's, Meryl is very much a part of this movie," she said. "There are absolutely no flashbacks. No, because all the 'flashbacks' are of Lily playing Donna. So it's a prequel and a sequel, and so you see Meryl is very much a part of it."
"I'm pretty sure they were staying at the same place when Cher and Meryl were in town to shoot," Seyfried said. "I just pictured sweatpants and movies. I don't know what really happened. I guess they went to the bar, but I just thought it was so cute that they were staying in the same place."