Credit: Gary Gershoff/WireImage; Jon Kopaloff/FilmMagic

He may be twice her age, but according to E! News, that didn't stop Ryan Phillippe, 36, from hooking up with Demi Lovato, 18.

"It was really hot and heavy for a while," a source tells the site.

An insider confirms the news to Us Weekly, pointing out that "they were just hooking up" and "it was nothing serious."

E! News adds that the "Skyscraper" singer didn't know the Lincoln Lawyer actor was going to be a father again "until it basically happened. It kind of threw her for a loop."

(Phillippe's ex-girlfriend Alexis Knapp welcomed daughter Kai at L.A.'s Cedars-Sinai Medical Center on July 1; the actor has yet to take a DNA test to prove his paternity. Phillippe also has two children -- Ava, 11, and Deacon, 7 -- from his first marriage to Reese Witherspoon.)

After seven months together, Phillippe split with on-off girlfriend Amanda Seyfriend in June 2011. Lovato's past loves include Wilmer Valderrama, Joe Jonas and Trace Cyrus.