Hayden Panettiere and Brian Hickerson Forced to Keep Rekindled Romance Quiet

Hayden Panettiere and Brian Hickerson kept their rekindled romance to themselves in order to protect her career ... TMZ has learned.

A source close to Brian tells TMZ ... the couple didn't break up as many times as the public was led to believe -- and were actually together while she was promoting her memoir "This Is Me: A Reckoning" in the months before her death.

Our source says Hayden was advised by her team to distance herself from Brian, at least publicly, even though they were still romantic in private.

The reason ... Hayden's camp felt Brian was "bad for business" while she was promoting her book. We're told their complicated history was perceived as being bad for book sales -- but they were still in constant communication during the promotional tour.

We're also told Hayden basically became a member of the Hickerson family in recent years ... especially in light of the fact that she didn't have a close relationship with her own family.

A separate source close to Brian confirms they've been dating consistently for years ... and reports of them going their separate ways were false and were spread as a way to help her career.

Our source saw Hayden and Brian together multiple times this past year in Los Angeles and South Carolina ... mainly at friends' gatherings.

In fact, we're told people in their inner circle would laugh at media headlines claiming they'd split ... because they were still a couple.

Hayden was actually asked about her relationship status with Brian, who once served jail time for pleading no contest to two charges stemming from alleged violent incidents with Hayden, back in May ... and she laughed at the idea of marrying him.

And, back in March, Brian helped Hayden -- who was using crutches to get around -- get to her car at LAX ... though we were told at the time the two were just friends.

As you know ... Hayden died Sunday afternoon at an Airbnb in Greenville, South Carolina ... and our sources say Brian was in town as well. Unclear what they were doing in Greenville, but we're told they visit the area often.

First responders were called to a residence Sunday afternoon around 1:50 PM to help a woman who was in cardiac arrest. They attempted life-saving measures, but ultimately pronounced Hayden dead at 2:32 PM.

Dispatch audio mentioned a possible overdose, and Narcan was found in the apartment where she died -- the medication is typically used for opioid overdose victims.