Swift says the tumor was discovered while her mother was undergoing treatment for cancer.
"While she was going through treatment, they found a brain tumor. And the symptoms of what a person goes through when they have a brain tumor is nothing like what we've ever been through with her cancer before," Swift says. "So it's just been a really hard time for us as a family."
Swift, who calls her mom her "guiding force," says her current limited touring schedule is due to her desire to be there for her mom during this difficult time.
"We don't know what is going to happen. We don't know what treatment we're going to choose. It just was the decision to make at the time, for right now, for what's going on," Swift says.
"Who am I supposed to talk to? What am I supposed to do? If there's no you?" Swift sings in the song.
Swift first revealed in mom's cancer diagnosis in 2015. In early 2019, during the filming of Miss Americana, a documentary about Swift's life and career, Andrea Swift's cancer returned.
"It's taught me that there are real problems and then there's everything else," Swift wrote in a 2019 essay for Elle. "My mom's cancer is a real problem. I used to be so anxious about daily ups and downs. I give all of my worry, stress, and prayers to real problems now.
Miss Americana will be released Jan. 31 on Netflix.