"Catherine has proactively checked into a health care facility," her rep, Cece Yorke, tells PEOPLE. "Previously Catherine has said that she is committed to periodic care in order to manage her health in an optimum manner."
Meanwhile, a source tells PEOPLE that Zeta-Jones was planning to return to treatment as a way for doctors to monitor her medication. "There was no big problem," says a friend. "This was just a good time to do it. She is in between projects. This has always been part of the plan. She would manage her health. She is vigilant about it."
In 2011, the actress completed a stint in a Connecticut mental-health facility for the disorder, which can cause a cycle of high and low moods, often with episodes of depression.
"This is a disorder that affects millions of people and I am one of them," Zeta-Jones, 43, told PEOPLE at the time. "If my revelation of having bipolar II has encouraged one person to seek help, then it is worth it. There is no need to suffer silently and there is no shame in seeking help."
Nothing seemed amiss a week ago when Zeta-Jones and husband Michael Douglas walked the carpet at the Film Society of Lincoln Center’s 40th Annual Chaplin Award Gala honoring Barbra Streisand. When asked if they were enjoying a perfect date night, he smiled and responded, "Oh yes. Yes, this is pretty cool."
On Saturday, Douglas stepped out alone at the White House Correspondents Dinner.
Next up for Zeta-Jones is the movie Reds 2, out July 19, costarring Bruce Willis and Anthony Hopkins. Douglas is headed to Cannes in a few weeks for his upcoming HBO biopic Behind the Candelabra, costarring Matt Damon.
TMZ first reported Zeta-Jones's return to treatment, quoting a source who said, "It's maintenance."