The actress, who's expecting twins in January, was preparing for a baby shower in Los Angeles on Saturday when the contractions hit, Cagle says.
"They hooked her up to a fetal monitor," he explains, "and the contractions stopped, but the doctors have said, 'You're on bed rest.' It looks like she will be subjected to bed rest for the rest of the pregnancy."
As for the baby shower, it came to her, Cagle says: "A bunch of friends and family showed up on Sunday. And so she's feeling good. She's feeling good enough to receive visitors.
"This is not an uncommon. I think 75 percent of women carrying twins will go into preterm labor of some sort."
Roberts is expecting a boy and girl, he notes.
But the bed rest "is a big deal" for Roberts, Cagle continues, "because this is a big season for her."
She has two movies coming out in December, and the publicity mills are already grinding.
One is called "Closer" and the other is the "Oceans 12" sequel.
"She was planning for this," Cagle continues, "because they knew she would be about nine months pregnant when these movies were being released. So she had planned to do a couple of big interviews by satellite, and wasn't going to be out there shouting from the rooftops.
"It's too bad for the studios, though, because if she's on bed rest, she won't do a lot of promotions, and she is normally good at walking the red carpet. …She is a great interview subject."
In general, Cagle concluded, "This has been a good pregnancy. She worked in the beginning of it. She finished her two movies. Then she laid low," as she should have.