HOT on the heels of JENNIFER ANISTON’s surprise engagement to JUSTIN THEROUX, her mom NANCY DOW dropped a bombshell – she’s going to be a grandmother!

“Nancy said that Jen’s pregnant and that she and Justin would have a wedding soon in Mexico. She’s so beside herself over the news that she couldn’t resist spilling the beans,” said an insider.

“Only a few days after Jen confirmed she and Justin are engaged, Nancy squealed to a few friends and family members, ‘I can’t wait – I’m finally going to be a grandmother!’ ”

Actor/screenwriter Justin proposed to Jen on Aug. 10, his 41st birthday, over a romantic dinner at a New York City restaurant.

And as The ENQUIRER revealed, the 43-year-old actress is huddling with her lawyers to draw up a prenuptial agreement to protect her $120 million fortune.

“Jennifer and Justin are throwing together a shotgun wedding,” added the insider. “They plan to tie the knot in one of Jen’s favorite vacation locales, a beach in Mexico, and they’re going to do it before the end of the year.

“Nancy said that Jen doesn’t want to be showing too much on her wedding day.”

Jennifer appeared to be showing a slight bump already  as she filmed her latest flick, “We’re the Millers,” in Wilmington, N.C., on Aug. 17.

Jennifer and her 76-year-old mom have had a rocky relationship since 1996 after they famously fell out over Nancy discussing her daugh­ter’s personal life in a TV interview.

They reconciled after the former “Friends” star’s split from Brad Pitt in 2005. Since then, Jennifer has quietly provided financial support to her ailing mom, who fractured her hip and femur when she fell in early 2010.

Nancy also suffered a devastating stroke in September 2011, and as The ENQUIRER reported a few months after that incident, Jennifer promised her recovering mom a bundle of joy was coming soon.

And while Jennifer banned her mother from her splashy star-studded 2000 wedding to Pitt, Nancy will be an honored guest at Jennifer and Justin’s nuptials, said the source.

“Justin has been instrumental in smoothing over relations between Jen and Nancy,” said the insider. “He even calls Nancy once a week to see how she’s feeling.

“He always tells her, ‘Jennifer loves you so much, and so do I.’ Jen knows that her mom’s health is deteriorating. So she’s thrilled to be sharing her wedding and baby happiness with her. Like Nancy told her friends, ‘A grandchild is the best gift a grown child can give their mother.