The duo met on the set of the 1983 horror movie The Hunger and went on to quietly date for several years.
While they eventually split, Susan says she was able to reconnect with David in the months before his death in 2016.
"I was fortunate enough to be closer to him right before he died, the last couple of months. He did find me again. We talked to each other and said some things that needed to be said," Susan told You magazine.
Susan revealed that the last time she saw him was the premiere of his musical Lazarus the month before his passing, adding, "I was so fortunate to be able to see him when he told me what was going on with him."
After attending the premiere, Susan traveled to the Greek island of Lesbos to highlight its refugee crisis, and David ended up giving her a call.
"I wasn't sleeping and I knew that I had to get up early to start meeting the boats as they came in, so I took some Ambien, a pretty strong sleep aid. And I had this dream that David had called me and that we'd had this conversation," Susan explained.
She continued, "As I hung up I thought [in the dream]: 'Nobody's going to believe me, that David Bowie called me in Lesbos.' Then later, I thought: 'Did he actually call me?' And I went to my phone and he had."
Unfortunately, Susan says she has "no recollection" of what the conversation was about because of the sleep aid.
The duo were not able to connect again before David's passing the following week, making it "so frustrating" for Susan to not know what he had said.
And while she may never know the meaning behind their final conversation, Susan says she still keeps in touch with David's wife, Iman.
"I love his wife Iman, someone who was so equal in stature [to him]. That was clearly who he was destined to be with. I was so happy that she was with him through all of that. And I've kept in touch with her," Susan noted.