HEATHER LOCKLEAR has gone from super-sexy to scary-skinny!

The former “Melrose Place” beauty recently went on a “no carbs, no sugar” diet to get ready for her new role on the TNT law series “Franklin & Bash,” but she took things WAY too far, say pals.

“Heather looked great when she initially met with the stars and the producers of the show, so you can imagine how shocked they were when she showed up looking so rail-thin,” said a set insider. “She’s never been overweight and never had to watch what she eats, but like all actresses – especially the older ones – she’s insecure.

“Heather wanted to make sure her return to TV was memorable, so she cut out all carbs and sugar and the pounds just dropped off. But she’s gone too far, and even (her daughter) Ava, begged her to stop losing weight.”

The 51-year-old actress – who’s 5-foot-5 and normally weighs as much as 120 pounds – is barely tipping the scales at 100, says the source.

 Ava, 15, was so concerned about her mother’s health that she asked her dad – Bon Jovi rocker Richie Sambora – to step in.

“Heather admitted she got carried away and blamed it on the pressure to stay thin in Hollywood,” said the insider.

The gorgeous blonde, who plays new law firm partner Rachel King on “Franklin & Bash,” joins former “Saved by the Bell” hunk Mark-Paul Gosselaar and “Clueless” actor Breckin Meyer for the TNT series’ third season.

And there may be more to her weight loss than she’s admitting.

 Another source says that although Heather is happily dating Beverly Hills plastic surgeon Dr. Marc Mani, she was devastated when former fiancé Jack Wagner con­firmed in mid-May that he’d been romancing fellow soap star Ashley Jones for more than a year.

Heather and Jack were together for four years before the actress abruptly called off their engagement in 2011.

“Now Heather’s convinced Jack was sneaking around while they were still en­gaged,” revealed the source.

“Things have been going well for Heather lately, but pals know she’s going to treat Jack’s admission about him and Ashley as a huge blow to her self-esteem.”