Of course, it's not just any girlfriend ... it's Jennifer Lopez!
The 49-year-old actress looked sad during a recent beach event held by her 9-year-old son Samuel's school.
Samuel is just one child she shares with Affleck. The former duo also has two daughters, 15-year-old Violet and 12-year-old Seraphina.
While Garner and Affleck do an excellent job at co-parenting, the task has been left up to the 13 Going On 30 actress this week since Ben is off in Vegas for work.
The usual cheerful Garner didn't look pleased during her son's school party at a Los Angeles beach on Wednesday.
She was photographed crossing her arms and looked aloof as fellow mothers chatted away.
At one point, she steered away from the parents and was seen staring out into the water all by herself.
After spending some time at Samuel's school event, Garner made it clear she had enough.
Grabbing her son's hand, the two made their way past photographers and into their car to go home.
Garner held her head down the whole time and covered her face in large sunglasses.
It's an interesting move considering she was out and about last week, and looked like no one could dim her shine.
Her mood change might have something to do with recent reports alleging J Lo is "moving in" with Affleck.
On Friday, the Love Don't Cost a Thing singer was caught visiting at least one school near Affleck's Southern California home.
Fans began to wonder whether this move signified that J Lo was ready to blend her family with Affleck's.
As it stands, her 13-year-old twins Max and Emme -- who she shares with ex-husband Marc Anthony -- attend school in Miami.
Prior to the move in news, sources told E! that Garner has no interest in her ex's reunion with J Lo.
The actress "doesn't want to be a part of the circus or media attention in any way," the insider revealed.
"She's trying to live her life and raise her kids and the last thing she wants to be doing is be dealing with Ben's love life," the source revealed, adding, "Her focus is always on the kids' happiness and Ben being a good father."