Britney Spears is upsetwith her father Jamie followinghis alleged altercationwith her13-year-old son Sean Preston, a source close to the singer tells PEOPLE.

“Both of Britney’s boys are very well-behaved and well-mannered. What happened with Jamie and Sean was very unfortunate and definitely not right,” says the source, who adds that their alleged fight on Aug. 24 began after Sean “locked himself in a room” after his grandfather asked him to do something “he wasn’t interested in doing.”

Jamie, 67, “got very angry,” andaccording toThe Blast, then broke down the door. “There was physical contact that made Sean scared and upset,” says the source. “Britney got upset as well and ended their visit with Jamie.”

The 37-year-old singer — who now shares about 10 percent of custody of her kids withex-husband Kevin Federline— “can’t believe that her dad would jeopardize her relationship with her boys,” adds the source. “Britney is always terrified that she will lose custody.”

Jamie Spears; Britney Spears.Lester Cohen/Getty Images

Britney Spears

A rep and a lawyer for Spears did not immediately respond to a request for comment from PEOPLE. Jamie also did not respond to calls for comment.

The day after the alleged altercation, Federline, 41, filed a police report at the Ventura County Sherriff’s station with his lawyer Mark Vincent Kaplan. “Kevin got extremely angry when he heard what happened,” says the source. “His reaction does seem appropriate.”

According to Kaplan’s testimony as recorded in the police report, Spears’ 12-year-old son Jayden was also present during Jamie and Sean’s fight.

Britney Spears/Instagram

Britney-Spears-sons

“Britney did the right thing and removed the children and took them out of there,” Kaplan previously told PEOPLE, adding that Federline still wants his sons to have “continuing and meaningful contact with their mother.”

Kaplan confirmed to PEOPLE earlier on Tuesday that Spears and Federline came to a new child custody arrangement last week which gives Spears 10 percent of custodial rights and Federline 90 percent. Kaplan tells PEOPLE Spears’ visits are required to be supervised.

Earlier this year, Spears checked into an “all-encompassing wellness treatment” center followingJamie’s “life-threatening” colon rupturelast fall.

While gearing up for a return to the Las Vegas strip with her showDominationafter taking her wildly successful Vegas showBritney: Piece of Meon tour over the summer, Spears announced that she wastaking an indefinite breakfrom work in January to support her father.

In 2008, a Los Angeles court named Jamie permanent conservator of Spears’ affairs. He was also named permanent co-conservator of her estate, along with an attorney, Andrew Wallet.

source: people.com