Ben and Jen were at the Big Sky Resort in Montana, super close to Yellowstone National Park. They were staying at the same resort and were driving around together ... Ben was driving and J Lo was riding shotgun.

They also flew out of Montana together Sunday from Bozeman, Montana to L.A. When they arrived in L.A. at the Signature Terminal at LAX, they took an SUV together and drove to her house in Bel-Air. All this raises the question, of course ... are they back together?

There's certainly a lot of smoke. The 2 were hanging out together in Los Angeles early last week as well. If you were placing bets, bet on them being reunited as a couple.

Our sources connected to both J Lo and Ben have told us they're just friends ... they've actually been fairly close over the years since their breakup.

They certainly have a lot in common now ... they both recently broke off their relationships.

It was back in 2004 when the two split, and we haven't known either of them to be romantically linked since. Of course, both have gone on to have other serious relationships and raise children as well.

A source close to J Lo told us she keeps her exes close, just look at her relationship with Marc Anthony over the years. The source also told us she's seen Alex Rodriguez since they ended things as well and that the two remain friends. However, it's rare that only J Lo and one of her exes ever hung out without kids in tow, and from what we've learned, Ben and Jennifer were solo in Montana.