Jessica Simpson, who has a history of stalking ex-boyfriend John Mayer, is reportedly fuming over the musician’s rumored current romance Cameron Diaz. The relationship also has Cam’s ex Justin Timberlake green with envy.

An US Weekly source says Jess is “really jealous” that ex John Mayer is seeing Cameron Diaz.

“She had her mouth open a mile wide when she found out,” says the insider. “She just freaked.” Simpson, 27, parted ways with -Mayer, 29, in May after dating him on and off for seven months, and “she thinks about him all the time,” says the source. “She is so not over him.”