JENNIFER ANISTON might have a bun in the oven – or just an extra burrito in her belly!

The 43-year-old beauty ditched her rig­orous diet and exercise routines during a Mexican vacation with fiancé Justin Theroux and chowed down to her heart’s content, say sources.

After indulging in yummy enchiladas and frosty margaritas during the day and high-fat guacamole and chips at night, the former “Friends” star tried to cover up her paunchy midsection with an over­size T-shirt.

Now friends are wondering whether Jen’s long-time baby dream has come true, or whether she simply packed on a few extra pounds during her 10-day getaway.

“Jen really let herself go while she was in Mexi­co,” disclosed an insider.

“She stuck to her rigid routine of daily power yoga, jogging, Pilates and circuit training all year long. So when she got the opportunity to kick back with the love of her life, all she wanted to do was take it easy. She forgot about her strict diet of steamed fish and green juices.”

During their stay at a private villa in Cabo San Lucas, Jen and Justin, 41, hosted close pals John Krasinski of “The Office” and his wife, actress Emily Blunt, along with late-night TV host Jim­my Kimmel and his fiancée, Molly McNearney.

At one point, Jen cast an envious expression as she eyed Molly’s smoking-hot body.

The lovebirds had a private chef on call 24 hours a day and noshed on high-calorie goodies when­ever the mood struck them, said the source.

“Jen thought nothing of ordering up bowls of fresh guacamole with fried chips in the middle of the night, and then gorging on fish tacos and refried beans in the daytime,” said the source.

“She ate pancakes in the mornings and ex­tra large burritos for dinner. After packing in all that stuff, it’s no wonder she’s got friends wondering if she’s pregnant!”

While sources say the couple’s holiday ended on a high note, it got off on the wrong foot.

As The ENQUIRER reported last week, Jen and actor/screenwriter Justin were putting the finishing touches on their wed­ding plans when they began quarreling over her insecurities and their opposing lifestyles. They smoothed things over, but Jen is still dealing with her ailing 76-year-old mother Nancy Dow’s medical issues. Mother and daughter have long had a stormy relation­ship, and as The ENQUIRER reported shortly after Jen’s engagement in August, her mom had started blabbing to friends that she was soon going to be a grandmother.

“After all the stress Jen has been through, it would thrill her to pieces if she’s finally conceived,” said the source.

“But if not, Jen and Justin are glowing from the vacation of a lifetime. After the rocky start, it blossomed into exactly the type of carefree getaway Jen needed to recharge her batteries.”