A source close to Federline told E! Online that he had shared his belief with friends that Spears changed dramatically after she became a mother.
"He feels like Britney never was the same after she had their kids," the source said. "Even after the first baby, she had wild mood swings and her temper would go off at the drop of a hat.
"It got worse after her second pregnancy. Kevin didn't want another baby so soon, not because he didn't want more kids, but because her temper was so unstable when she was pregnant," the source continued. "After she came home from the hospital the second time, she just seemed really unhappy all of the time. He said, 'Nothing made her happy anymore. She was just angry or sad all the time.'"
Experts have said that Spears' recent spate of erratic behavior could have been motivated by postpartum depression, which can set in anytime during the first year after birth and affects up to 80 percent of new mothers.
Kevin Federline is standing by his former woman.
The would-be rapper paid a visit to his estranged wife in rehab on Saturday with the couple's two sons, 17-month-old Sean Preston and 5-month-old Jayden James in tow, People reports. (According to Us Weekly, it was Spears' mom, Lynne, who brought the kids.)
It was Federline's second visit to Promises since Spears reentered the treatment facility last Wednesday.
He previously visited Spears on Friday, bringing her cigarettes and photographs of their children, a source told E! News.