Celebrity Gossip's

Jessica Simpson is so mad that her ex John Mayer is hooking up with Cameron Diaz! Us Weekly is reporting that a source close to Jess said the singer is "really jealous" and can't get over it.

"She had her mouth open a mile wide when she found out,” says the source. “She just freaked.” John dumped Jess in May, after a seven-month fling, and Jessica is still reeling. "She thinks about him all the time,” the spy said. “She is so not over him.”

And Jessica might be waiting a while if she's thinkiing about swooping back in when John and Cam to break up. The new couple "looked very cozy" just last week at a club in NYC.