With rumors swirling that hubby ASHTON KUTCHER cheated on her, DEMI MOORE’s weight has plummeted!

The 5-foot-5 actress now weighs 99 pounds, sources say, while 15- years-younger Ashton has been accused of seducing a gorgeous young blonde at a California hotspot.

“Ashton was reportedly seen with a blonde named Sara Leal in San Diego,” revealed an insider. “She ran into him at the Hard Rock and they hit it off!

“At first it was no big deal because women run up to Ashton all the time. But pals of Sara are saying their accidental meeting led to a night in Ashton’s room. They’re saying that they had a ‘good’ time and Sara’s hoping to see Ashton on a regular basis.”

The 33-year-old actor, who’s replaced Charlie Sheen on the hit CBS sitcom “Two and a Half Men,” was partying with a group of pals, including his “That ’70s Show” co-star Danny Masterson, and actors Eric Balfour and Michael Pena, sources say.

Late on Sept. 25, the foursome hit Fluxx nightclub, where they downed vodka and tequila. A scruffy-looking Ashton, chain-smoking and wearing a floppy fedora, was overheard saying that none of the women there were “hot-tub worthy,” a source said.

“If Demi had heard him say that, she would have hit him,” said the source.

As for reports that he hooked up with Sara, Ashton “has pooh-poohed the whole thing,” the source added. “He says they just hung out.”

But it’s not the first time Ashton has been accused of cheating on 48-year-old Demi since their 2005 marriage.

In September 2010, he allegedly cheated twice with 21-year-old Demi look-alike Brittney Jones, and sources close to the “Ghost” beauty say she’s never recovered from the scandal.

“Demi has been on high alert since the Brittney cheating scandal,” a source close to the actress told The ENQUIRER. “She was devastated by it.”

Desperate to hang on to her man, “Demi started working out more and eating less,” said the source.

“She thinks she looks good, but she doesn’t. She’s lost way too much weight.”

The mother of three appeared shockingly thin in a form-fitting dress at a Hollywood event on Sept. 23, and a pair of black leather pants seemed to be falling off her tiny frame while she promoted her Lifetime movie “Five” in New York City on Sept. 26.

“This is yet another nightmare come true for Demi,” said the source.

After the rumors broke, the couple were spotted entering the Kabbalah Centre in LA for the Jewish Holidays after both made some unorthodox Tweets for the tech-savvy duo.

“Ashton is ot saying much about the cheating rumors or Sara, but he did say he likes San Diego a lot and will spend more time there when he can, which has only helped confirm Demi’s fears.”