The 18-year-old singer reportedly began seeing the 36-year-old a couple of months ago, though their relationship is said to have lost its intensity following the recent birth of Phillippe's daughter.
"It was really hot and heavy for a while," a source told E! Online.
The insider went on to say that Lovato was not sure that Phillippe was going to be a father "until it basically happened".
"It kind of threw her for a loop," the source said.
Phillippe previously dated Mamma Mia! actress Amanda Seyfried before splitting from the 25-year-old in late May.
The Lincoln Lawyer star was married to Reese Witherspoon from 1999 to 2007. He has two children, Ava, 11, and Deacon, 8, with the Legally Blonde actress.