Calista Flockhart’s dramatic collapse on the “Ally McBeal” set has set alarm bells ringing for her worried pals. The 5-foot, 5 1¼2-inch, 98-pound actress has lost weight, quit exercising and drinks nerve-jangling double espressos all day long, sources say.

Frantic friends are encouraging her to eat more and resume her exercise routine. But the 36-year-old actress insists she’s fine. “Calista is headed for disaster!” an insider predicted. “If she doesn’t put on some weight soon, she’ll seriously damage her body — or even die!”

Just a few months ago, “Calista had added six pounds to her 104-pound frame — and she was doing great,” said a source. “She was eating three meals a day — a bagel with cream cheese for breakfast, a sandwich or salad for lunch, then a dinner of pasta and a salad.

“She even had enough energy to jog with her trainer.”

Then the stressed-out star fell off the wagon as she worried about her show’s sinking ratings. “Calista’s weight started going down and she began looking exhausted,” said the insider.

“Calista was working 15- to 18-hour days and neglecting everything else — her exercise, her friends and, most importantly, food. She just stopped eating! Before long, the six pounds that she’d gained were gone.”

Now she’s “down to a dangerously low 98 pounds,” says an eyewitness. “When she goes out to restaurants, she barely has a bite to eat. And she drinks A LOT of coffee! Every day, like clockwork, she goes to the Starbucks in Brentwood near her home and orders FOUR double espressos! And I haven’t seen her working out for months.”

“With this kind of extreme weight loss, Calista Flockhart could be jeopardizing her health,” said weight loss expert Dr. Michael Steelman. “It can cause precipitous drops in blood pressure leading to dizziness, fainting and even heart attacks.

“Drinking a lot of coffee only adds to these dangers because it can increase dehydration and provoke heart arrhythmias.”

A depleted Calista collapsed on the “Ally” set December 13.

“Things were going as usual when Calista fell over like a straw in the wind,” said the source. For a split second, the place went silent. Then someone shouted, ‘Call an ambulance!’

“When she came around, she laughed nervously and said, ‘Oops! I guess this flu bug has really gotten to me.’

“But we all knew Calista’s body had simply given out on her.

“Paramedics took Calista to a local hospital where doctors discovered she was not only dehydrated, but exhausted from stress.”

Just a few days after she collapsed, Calista came out of her house to walk her dog Webster.

“She had no makeup on, her hair looked stringy and she was ghostly pale with dark shadows under her eyes,” said the eyewitness.

Added the source: “Everyone on ‘Ally’ tries to get Calista to eat more, look after herself and exercise again.

“Calista insists that she’s fine. But it’s obvious that the stress of the show is running her down.”