Inside Louis Tomlinson's Custody Battle with Ex Briana Jungwirth: 'She Thought It Was Going to Be a Real Relationship,' Source Says

Louis Tomlinson and Briana Jungwirth‘s disagreement over the custody of their child may have been sparked by her jealousy over his new relationship, a source close to the stylist tells PEOPLE.

Tomlinson, 24, filed for 50/50 joint physical and legal custody of 5-month-old Freddie Reign after visits with his son became increasingly erratic, a source close to the singer tells PEOPLE, prompting him to act. The cordial co-parenting relationship became complicated once he introduced their son to his new girlfriend, actress Danielle Campbell, says the Jungwirth source.

“Before Briana got pregnant, she was bragging to friends about how serious she was with Louis,” says the source. “She thought it was going to be a real relationship.”

“When Louis started dating Danielle, Briana was upset and that’s when the drama started,” adds the source.

The former One Direction singer and Campbell were rumored to be dating back in December – while Jungwirth was still pregnant – but didn’t confirm their relationship until April.

“When she was pregnant he was very supportive and paying for all her medical needs,” says the source. “He bought her a house, but she wanted something bigger.”

Since welcoming his first child in January, Tomlinson looks to be a devoted and loving father, posting several sweet pictures with Freddie on social media and taking him along on errands, but his relationship with Jungwirth hasn’t been so smooth behind the scenes.

The insider previously told PEOPLE Tomlinson and Jungwirth had an “interim agreement” regarding Tomlinson’s access to the baby, and that the former couple had worked out long-term custody and child support agreements.

“Briana really tried to make the relationship work, and she became jealous when he moved on,” says the source.

A rep for Tomlinson had no comment.