KENDRA WILKINSON took extreme measures to get her hot body back after giving birth when she saw how quickly KOURTNEY KARDASHIAN lost her baby bulge!

In her just-published book, “Being Kendra: Cribs, Cock­tails & Getting My Sexy Back,” the 26-year-old former Playboy Playmate reveals she was so desperate she started purging after eating!

She writes that Kourtney “was back to her pre-baby body while I hadn’t lost a pound…I even tried throw­ing up food, but bulimia wasn’t my thing. I gagged myself with my fingers and the food came back up. I was desperate for something, anything to work.”

Meantime, Kourtney, 32, had seemed to shed weight with ease, dropping 45 pounds and show­ing off her 95-pound bikini body in photos.

“I would see pictures of (Kourtney) in the magazines, and her body was already back to normal after a few months,” recalls Kendra, who adds she was feverishly work­ing out twice a day.

“I kept thinking, ‘What is wrong with me? Why can she do it and I can’t?’ ”

A friend of Kendra’stold The ENQUIRER: “She was so upset because they had kids just three days apart.”

Even five months after Kendra gave birth to Hank Baskett IV via C-section, she still weighed 140 and considered hav­ing lipo surgery. But doctors finally discovered the real reason the 5-foot-4 beauty was having so much trouble.

“Kendra’s doctor found an undiag­nosed health issue –low thyroid levels – was stopping her from losing weight,” explained the friend.

“She took a thyroid medication, lost three pounds in two weeks and was on her way!”