It looks like a famous ex may be what finally came between Jessica Simpson and Tony Romo.

Sources tell Radar Online that Simpson was keeping in touch with John Mayer, whom she dated in 2006, and Romo wasn’t thrilled when he found out about it.

“They were hanging out and he picked up her phone and she got defensive about it. Tony found messages from John and went ballistic,” a source says.

“Tony dumped her right then. He walked out and that was it.”