Discovery

Jessica Simpson is not a happy camper.

The singer is reportedly “really jealous” that ex John Mayer has upgraded to Cameron Diaz.

A source tells Us Weekly, “She had her mouth open a mile wide when she found out. She just freaked.”

Finding out your ex is dating Cameron Diaz has got to suck.

John and Jessica split in May after dating off and on for seven months, but she apparently still has feelings for him.

Adds the source, “She thinks about him all the time. She is so not over him.”

Meanwhile, Mayer has had no problems moving on. He’s been seen all over NYC with Cameron the past couple of weeks.

A source close to the new couple adds, “They’re happy.”

Ouch. Oh, well at least Simpson always has her hairstylist Ken Paves.